lib: elf: Move the generic elf loading/validating functions to lib

Move the generic elf loading/validating functions to lib/
so that they can be re-used and accessed by code existing
outside cmd.

While at it remove the duplicate static version of load_elf_image_phdr
under arch/arm/mach-imx/imx_bootaux.c.

Signed-off-by: Keerthy <j-keerthy@ti.com>
Suggested-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>
Reviewed-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>