OvmfPkg: PlatformPei: take no-exec DXE settings from the QEMU command line

OvmfPkg: PlatformPei: take no-exec DXE settings from the QEMU command line

Control them with:

  -fw_cfg name=opt/ovmf/PcdPropertiesTableEnable,file=no.txt \
  -fw_cfg name=opt/ovmf/PcdSetNxForStack,file=yes.txt

where the contents of the text files can be

  [0nN1yY](\n|\r\n)?

The macro trickery is not optimal, but it is caused by PcdSetBool(), which
is itself a macro, and can only take open-coded PCD names (ie. no
variables, like function parameters).

Cc: Jordan Justen <jordan.l.justen@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18471 6f19259b-4bc3-4df7-8a09-765794883524