package/linux-firmware: also install into images for early loading support

package/linux-firmware: also install into images for early loading support

Some drivers request their firmware very early when built into the kernel,
even before the initramfs is mounted - So the only way to provide firmware
for those drivers is to include them directly in the kernel with the
CONFIG_EXTRA_FIRMWARE option.

An example of this is the uC firmware for modern Intel GPUs.

Conceptually you can point CONFIG_EXTRA_FIRMWARE to
${TARGET_DIR}/lib/firmware, but then you cannot remove the firmware from the
initramfs and pay the size cost twice (inside the kernel + in initramfs), so
instead also install linux-firmware to the images dir, similar to how we do
it for intel-microcode.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>