efi_loader: rework efi_search_obj

EFI_HANDLEs are used both in boottime and in runtime services.
efi_search_obj is a function that can be used to validate
handles. So let's make it accessible via efi_loader.h.

We can simplify the coding using list_for_each_entry.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>