OvmfPkg/build.sh: Support --enable-flash switch

OvmfPkg/build.sh: Support --enable-flash switch

If this argument is used, then when QEMU is run, the -pflash
parameter will be used with QEMU to enable QEMU's flash
mode.

It should be used before the 'qemu' argument, since it is
not a QEMU parameter, but instead it updates how build.sh
runs QEMU.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@14841 6f19259b-4bc3-4df7-8a09-765794883524