OvmfPkg: Add build-flag SECURE_BOOT_FEATURE_ENABLED

OvmfPkg: Add build-flag SECURE_BOOT_FEATURE_ENABLED

SECURE_BOOT_FEATURE_ENABLED is the build-flag defined when secure boot
is enabled. Currently this flag is used in below lib:
 - OvmfPkg/PlatformPei
 - PeilessStartupLib

So it is defined in below 5 .dsc
 - OvmfPkg/CloudHv/CloudHvX64.dsc
 - OvmfPkg/IntelTdx/IntelTdxX64.dsc
 - OvmfPkg/OvmfPkgIa32.dsc
 - OvmfPkg/OvmfPkgIa32X64.dsc
 - OvmfPkg/OvmfPkgX64.dsc

Cc: Erdem Aktas <erdemaktas@google.com>
Cc: James Bottomley <jejb@linux.ibm.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Min Xu <min.m.xu@intel.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>