linux: build after linux-firmware if enabled for early loading support

To support building in (a subset of) the linux-firmware files into the
kernel using the CONFIG_EXTRA_FIRMWARE option, we need to ensure that the
firmware files are installed before the Linux kernel is built, similar to
how it is done for intel-microcode.

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

linux/linux.mk

@@ -78,7 +78,8 @@ LINUX_MAKE_ENV = \
 
 LINUX_INSTALL_IMAGES = YES
 LINUX_DEPENDENCIES = host-kmod \
-	$(if $(BR2_PACKAGE_INTEL_MICROCODE),intel-microcode)
+	$(if $(BR2_PACKAGE_INTEL_MICROCODE),intel-microcode) \
+	$(if $(BR2_PACKAGE_LINUX_FIRMWARE),linux-firmware)
 
 # Starting with 4.16, the generated kconfig paser code is no longer
 # shipped with the kernel sources, so we need flex and bison, but