In order to revoke SRKs in platforms with AHAB we need to set a mask
during the signing/encryption process.
Create new TRUSTFENCE_SRK_REVOKE_MASK variable to export the
SRK_REVOKE_MASK variable required by the imx-boot signing script.
The revoke mask is not necessary for signing/encryption of other artifacts,
so set it by default to 0x0.
Signed-off-by: Gonzalo Ruiz <Gonzalo.Ruiz@digi.com>