efi_loader: carve out efi_load_image_from_file()

efi_load_image_from_file() should read via either of:

* EFI_SIMPLE_FILE_SYSTEM_PROTOCOL
* EFI_LOAD_FILE_PROTOCOL
* EFI_LOAD_FILE2_PROTOCOL

To make the code readable carve out a function to load the image via the
file system protocol.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>