efi_loader: do not use symbolic links

Symbolic links are not supported on all file systems, e.g. not on FAT. So
it is not wise to use them in our source tree.

Use a qualified path to refer to lib/efi_loader/efi_freestanding.c in
scripts/Makefile.lib instead.

Reported-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
[agraf: Fix build with O=]
Signed-off-by: Alexander Graf <agraf@suse.de>