spl: fit: Prefer a malloc()'d buffer for loading images

Fit images were loaded to a buffer provided by spl_get_load_buffer().
This may work when the FIT image is small and fits between the start
of DRAM and SYS_TEXT_BASE.

One problem with this approach is that the location of the buffer may
be manipulated by changing the 'size' field of the FIT. A maliciously
crafted FIT image could place the buffer over executable code and be
able to take control of SPL. This is unacceptable for secure boot of
signed FIT images.

Another problem is with larger FIT images, usually containing one or
more linux kernels. In such cases the buffer be be large enough so as
to start before DRAM (Figure I). Trying to load an image in this case
has undefined behavior.
For example, on stm32mp1, the MMC controller hits a RX overrun error,
and aborts loading.
    _________________
   |    FIT Image    |
   |                 |
  /===================\                        /=====================\
  ||      DRAM       ||                        |        DRAM         |
  ||                 ||                        |                     |
  ||_________________||  SYS_TEXT_BASE         | ___________________ |
  |                   |                        ||     FIT Image     ||
  |                   |                        ||                   ||
  | _________________ |  SYS_SPL_MALLOC_START  || _________________ ||
  ||  malloc() data  ||                        |||  malloc() data  |||
  ||_________________||                        |||_________________|||
  |                   |                        ||___________________||
  |                   |                        |                     |

        Figure I                                       Figure II

One possibility that was analyzed was to remove the negative offset,
such that the buffer starts at SYS_TEXT_BASE. This is not a proper
solution because on a number of platforms, the malloc buffer() is
placed at a fixed address, usually after SYS_TEXT_BASE. A large
enough FIT image could cause the malloc()'d data to be overwritten
(Figure II) when loading.

          /======================\
          |        DRAM          |
          |                      |
          |                      |   CONFIG_SYS_TEXT_BASE
          |                      |
          |                      |
          | ____________________ |   CONFIG_SYS_SPL_MALLOC_START
          ||   malloc() data    ||
          ||                    ||
          || __________________ ||
          |||    FIT Image     |||
          |||                  |||
          |||                  |||

                 Figure III

The solution proposed here is to replace the ad-hoc heuristics of
spl_get_load_buffer() with malloc(). This provides two advantages:
    * Bounds checking of the buffer region
    * Guarantees the buffer does not conflict with other memory

The first problem is solved by constraining the buffer such that it
will not overlap currently executing code. This eliminates the chance
of a malicious FIT being able to replace the executing SPL code prior
to signature checking.

The second problem is solved in conjunction with increasing
CONFIG_SYS_SPL_MALLOC_SIZE. Since the SPL malloc() region is
carefully crafted on a per-platform basis, the chances of memory
conflicts are virtually eliminated.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>

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