roms: Allow passing configure options to the EDK2 build tools

roms: Allow passing configure options to the EDK2 build tools

Since commit f590a812c210 we build the EDK2 EfiRom utility
unconditionally.

Some distributions require to use extra compiler/linker flags,
i.e. SUSE which enforces the PIE protection (see [*]).

EDK2 build tools already provide a set of variables for that,
use them to allow the caller to easily inject compiler/linker
options..

Now build scripts can pass extra options, example:

  $ make -C roms \
      EDK2_BASETOOLS_OPTFLAGS='-fPIE' \
      efirom

[*] https://lists.opensuse.org/opensuse-factory/2017-06/msg00403.html

Reported-by: Olaf Hering <[email protected]>
Suggested-by: Laszlo Ersek <[email protected]>
Signed-off-by: Philippe Mathieu-Daudé <[email protected]>
Message-Id: <20190409134536[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Reviewed-by: Laszlo Ersek <[email protected]>
Reviewed-by: Igor Mammedov <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>