ArmPlatformPkg/ArmVExpress-FVP: enable UEFI Secure Boot

This allows the FVP target to be built with UEFI Secure Boot enabled,
by passing -D SECURE_BOOT_ENABLE to the build command line. Note that
this requires the Intel BDS, or you will not be able to enroll
certificates, since the ARM BDS does not provide a GUI to do so.

The FVP Base model is recommended in this case, since the certificate
store is kept in NOR flash.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Ryan Harkin <ryan.harkin@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18379 6f19259b-4bc3-4df7-8a09-765794883524