linux: add a comment about CONFIG_EXTRA_FIRMWARE requirement

Explain why the kernel needs firmware packages as build time
dependencies.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
(cherry picked from commit 8c4b55053e093bfaa88d93a9e3ad0ca241679bcd)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

linux/linux.mk

@@ -70,7 +70,11 @@ LINUX_MAKE_ENV = \
 	BR_BINARIES_DIR=$(BINARIES_DIR)
 
 LINUX_INSTALL_IMAGES = YES
-LINUX_DEPENDENCIES = host-kmod \
+LINUX_DEPENDENCIES = host-kmod
+
+# The kernel CONFIG_EXTRA_FIRMWARE feature requires firmware files at build
+# time. Make sure they are available before the kernel builds.
+LINUX_DEPENDENCIES += \
 	$(if $(BR2_PACKAGE_INTEL_MICROCODE),intel-microcode) \
 	$(if $(BR2_PACKAGE_LINUX_FIRMWARE),linux-firmware) \
 	$(if $(BR2_PACKAGE_WIRELESS_REGDB),wireless-regdb)