efi_loader: Re-factor code to build the signature store from efi signature list

The efi_sigstore_parse_sigdb function reads the uefi authenticated
variable, stored in the signature database format and builds the
signature store structure. Factor out the code for building
the signature store. This can then be used by the capsule
authentication routine to build the signature store even when the
signature database is not stored as an uefi authenticated variable

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>