cmd: elf: Reorder load_elf_image_phdr() and load_elf_image_shdr()

Move load_elf_image_phdr() and load_elf_image_shdr() to the beginning
of the cmd_elf.c so that forward declaration is not needed.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Tom Rini <trini@konsulko.com>