tools/firmware: update OVMF Makefile, when necessary

tools/firmware: update OVMF Makefile, when necessary

[ This is two commits from master aka staging-4.12: ]

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>
(cherry picked from commit b16281870e06f5f526029a4e69634a16dc38e8e4)

tools: only call git when necessary in OVMF Makefile

Users may choose to export a snapshot of OVMF and build it
with xen.git supplied ovmf-makefile. In that case we don't
need to call `git submodule`.

Fixes b16281870e.

Reported-by: Olaf Hering <olaf@aepfle.de>
Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>
Release-acked-by: Juergen Gross <jgross@suse.com>
(cherry picked from commit 68292c94a60eab24514ab4a8e4772af24dead807)