Automatic software update from TFTP server

The auto-update feature allows to automatically download software updates
from a TFTP server and store them in Flash memory during boot. Updates are
contained in a FIT file and protected with SHA-1 checksum.

More detailed description can be found in doc/README.update.

Signed-off-by: Rafal Czubak <rcz@semihalf.com>
Signed-off-by: Bartlomiej Sieka <tur@semihalf.com>

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+Automatic software update from a TFTP server
+============================================
+
+Overview
+--------
+
+This feature allows to automatically store software updates present on a TFTP
+server in NOR Flash. In more detail: a TFTP transfer of a file given in
+environment variable 'updatefile' from server 'serverip' is attempted during
+boot. The update file should be a FIT file, and can contain one or more
+updates. Each update in the update file has an address in NOR Flash where it
+should be placed, updates are also protected with a SHA-1 checksum. If the
+TFTP transfer is successful, the hash of each update is verified, and if the
+verification is positive, the update is stored in Flash.
+
+The auto-update feature is enabled by the CONFIG_UPDATE_TFTP macro:
+
+#define CONFIG_UPDATE_TFTP		1
+
+
+Note that when enabling auto-update, Flash support must be turned on.  Also,
+one must enable FIT and LIBFDT support:
+
+#define CONFIG_FIT		1
+#define CONFIG_OF_LIBFDT	1
+
+The auto-update feature uses the following configuration knobs:
+
+- CONFIG_UPDATE_LOAD_ADDR
+
+  Normally, TFTP transfer of the update file is done to the address specified
+  in environment variable 'loadaddr'. If this variable is not present, the
+  transfer is made to the address given in CONFIG_UPDATE_LOAD_ADDR (0x100000
+  by default).
+
+- CONFIG_UPDATE_TFTP_CNT_MAX
+  CONFIG_UPDATE_TFTP_MSEC_MAX
+
+  These knobs control the timeouts during initial connection to the TFTP
+  server. Since a transfer is attempted during each boot, it is undesirable to
+  have a long delay when a TFTP server is not present.
+  CONFIG_UPDATE_TFTP_MSEC_MAX specifies the number of seconds to wait for the
+  server to respond to initial connection, and CONFIG_UPDATE_TFTP_CNT_MAX
+  gives the number of such connection retries. CONFIG_UPDATE_TFTP_CNT_MAX must
+  be non-negative and is 0 by default, CONFIG_UPDATE_TFTP_MSEC_MAX must be
+  positive and is 1 by default.
+
+Since the update file is in FIT format, it is created from an *.its file using
+the mkimage tool. dtc tool with support for binary includes, e.g. in version
+1.2.0 or later, must also be available on the system where the update file is
+to be prepared. Refer to the doc/uImage.FIT/ directory for more details on FIT
+images.
+
+
+Example .its files
+------------------
+
+- doc/uImage.FIT/update_uboot.its
+
+  A simple example that can be used to create an update file for automatically
+  replacing U-Boot image on a system.
+
+  Assuming that an U-Boot image u-boot.bin is present in the current working
+  directory, and that the address given in the 'load' property in the
+  'update_uboot.its' file is where the U-Boot is stored in Flash, the
+  following command will create the actual update file 'update_uboot.itb':
+
+  mkimage -f update_uboot.its update_uboot.itb
+
+  Place 'update_uboot.itb' on a TFTP server, for example as
+  '/tftpboot/update_uboot.itb', and set the 'updatefile' variable
+  appropriately, for example in the U-Boot prompt:
+
+  setenv updatefile /tftpboot/update_uboot.itb
+  saveenv
+
+  Now, when the system boots up and the update TFTP server specified in the
+  'serverip' environment variable is accessible, the new U-Boot image will be
+  automatically stored in Flash.
+
+  NOTE: do make sure that the 'u-boot.bin' image used to create the update
+  file is a good, working image. Also make sure that the address in Flash
+  where the update will be placed is correct. Making mistake here and
+  attempting the auto-update can render the system unusable.
+
+- doc/uImage.FIT/update3.its
+
+  An example containing three updates. It can be used to update Linux kernel,
+  ramdisk and FDT blob stored in Flash. The procedure for preparing the update
+  file is similar to the example above.