roms/Makefile.edk2: define edk2-stable201905 network feature test macros

roms/Makefile.edk2: define edk2-stable201905 network feature test macros

Due to edk2 commits 631195044ff0 ("OvmfPkg: Update DSC/FDF to use
NetworkPkg's include fragment file.", 2019-05-16) and 157a3b1aa50f
("ArmVirtPkg: Update DSC/FDF to use NetworkPkg's include fragment file.",
2019-05-16), we'll have to pefix the HTTP_BOOT_ENABLE and TLS_ENABLE build
flags with "NETWORK_", for building edk2 at tag "edk2-stable201905".

Right now the new flags are no-ops, so we can insert them separately,
before advancing our edk2 submodule reference. We will remove the old
flags afterwards.

Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
Ref: https://bugs.launchpad.net/qemu/+bug/1831477
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>