efi_loader: set entry point in efi_load_pe()

Up to now efi_load_pe() returns the entry point or NULL in case of an
error. This does not allow to return correct error codes from LoadImage().

Let efi_load_pe() return a status code and fill in the entry point in the
corresponding field of the image object.

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