cmake: New list for post build commands that should run early

New cmake list, `post_build_patch_elf_commands`, will be prepended to
`post_build_commands` one, effectively making these commands to run just
after the ELF is created. It's particularly useful for operations that
patch the ELF generated, before other representations of it (such as
.bin) are created.

Signed-off-by: Ederson de Souza <ederson.desouza@intel.com>