firmware: Add firmware installation to modules_install, add firmware_install

For 'make modules_install', install any firmware required by
the modules which are being installed.

Also add a 'make firmware_install' target which doesn't depend on the
configuration, but installs _all_ available in-kernel-tree firmware into
$(INSTALL_FW_PATH), which defaults to /lib/firmware. This is intended
for distributors to make arch-independent (and config-independent)
packages containing firmware.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>