efi_loader: add signature verification functions

In this commit, implemented are a couple of helper functions which will be
used to materialize variable authentication as well as image authentication
in later patches.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
diff --git a/lib/efi_loader/Makefile b/lib/efi_loader/Makefile
index 9b3b704..eff3c25 100644
--- a/lib/efi_loader/Makefile
+++ b/lib/efi_loader/Makefile
@@ -44,3 +44,4 @@
 obj-$(CONFIG_GENERATE_SMBIOS_TABLE) += efi_smbios.o
 obj-$(CONFIG_EFI_RNG_PROTOCOL) += efi_rng.o
 obj-$(CONFIG_EFI_LOAD_FILE2_INITRD) += efi_load_initrd.o
+obj-y += efi_signature.o