linux: build after wireless-regdb if enabled for early loading support

To support building in the wireless regulatory database files (regulatory.db*)
into the kernel using the CONFIG_EXTRA_FIRMWARE option, we need to ensure that
the database files are installed before the Linux kernel is built.
The dependency is harmless if CONFIG_EXTRA_FIRMWARE isn't actually set.

Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
(cherry picked from commit de0f5ba17a15a87bc16a6d6aa318104510c38f9d)
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_WIRELESS_REGDB),wireless-regdb)
 
 # Starting with 4.16, the generated kconfig paser code is no longer
 # shipped with the kernel sources, so we need flex and bison, but