linux-firmware: don't put the firmware into the sysroot

There's no need to have linux-firmware in the sysroot. The sysroot won't
ever be used anyway as nothing needs the firmware at build-time, but this
saves us building a ~900MB sysroot (~300MB sstate tarball).

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>