configure: Only decompress EDK2 blobs for X86/ARM targets

The EDK2 firmware blobs only target the X86/ARM architectures.
Define the DECOMPRESS_EDK2_BLOBS variable and only decompress
the blobs when the variable exists.

See also: 536d2173 ("roms: build edk2 firmware binaries ...")
Suggested-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20191108114531.21518-2-philmd@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Reviewed-by: Luc Michel <luc.michel@greensocs.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>