bl1: add bl1_plat_handle_{pre,post}_image_load() ...

Just like bl2_, add pre/post image load handlers for BL1.  No argument
is needed since BL2 is the only image loaded by BL1.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

bl2: add bl2_plat_handle_pre_image_load() ...

There are cases where we need to manipulate image information before
the load.  For example, for decompressing data, we cannot load the
compressed images to their final destination.  Instead, we need to
load them to the temporary buffer for the decompressor.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

plat/common: move arch-agnostic fallback functions to C file ...

When we add a new callback, we need to duplicate fallbacks among
plat/common/{aarch32,aarch64}/platform_helpers.S  This is tedious.

I created a new C file, then moved 3 functions:
  plat_error_handler
  bl2_plat_preload_setup
  plat_try_next_boot_source

They are called from C, so I do not see a good reason to implement
them in assembly.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>