Force 2MB rom size

Suggested by Laszlo, upstream will be changing the default to 4MB, so
this is a preventative measure to ensure the rom size doesn't change
on the next rebase. Right now it's a no-op though

(cherry picked from commit 410bf18174)

edk2.spec

@@ -152,6 +152,7 @@ CC_FLAGS="${CC_FLAGS} --cmd-len=65536"
 OVMF_FLAGS="${CC_FLAGS}"
 OVMF_FLAGS="${OVMF_FLAGS} -D HTTP_BOOT_ENABLE"
 OVMF_FLAGS="${OVMF_FLAGS} -D NETWORK_IP6_ENABLE"
+OVMF_FLAGS="${OVMF_FLAGS} -D FD_SIZE_2MB"
 
 # ovmf + secure boot features
 OVMF_SB_FLAGS="${OVMF_FLAGS}"