tools/firmware: update OVMF Makefile

OVMF has become dependent on OpenSSL, which is included as a
submodule.  Initialise submodules before building.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>

tools/firmware/ovmf-makefile

@@ -16,6 +16,7 @@ all: build
 
 .PHONY: build
 build:
+	$(GIT) submodule update --init --recursive
 	OvmfPkg/build.sh -a X64 -b $(TARGET) -n 4
 	cp Build/OvmfX64/$(TARGET)_GCC*/FV/OVMF.fd ovmf.bin