efi_loader: signature: move efi_guid_cert_type_pkcs7 to efi_signature.c

The global variable, efi_guid_cert_type_pkcs7, will also be used in
efi_image_loader.c in a succeeding patch so as to correctly handle
a signature type of authenticode in signed image.

Meanwhile, it is currently defined in efi_variable.c. Once some secure
storage solution for UEFI variables is introduced, efi_variable.c may
not always be compiled in.

So move the definition to efi_signature.c as a common place.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>