package/rpi-firmware: fix config.txt installation

In commit 689b9ac439ab (package/rpi-firmware: rework boot/config file
handling), a last-minute fixup was made to handle the case where no
config.txt file was provided by the user.

However, the intermediate variable RPI_FIRMWARE_CONFIG_FILE was not used
consistently, and the old PACKAGE_RPI_FIRMWARE_CONFIG_FILE variable was
still used where RPI_FIRMWARE_CONFIG_FILE should have been used instead.

Signed-off-by: Grzegorz Blach <grzegorz@blach.pl>
Reviewed-by: Peter Seiderer <ps.report@gmx.net>
[yann.morin.1998@free.fr: blame myself for the mess; reword commit log]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

package/rpi-firmware/rpi-firmware.mk

@@ -28,9 +28,9 @@ define RPI_FIRMWARE_INSTALL_BIN
 endef
 
 RPI_FIRMWARE_CONFIG_FILE = $(call qstrip,$(BR2_PACKAGE_RPI_FIRMWARE_CONFIG_FILE))
-ifneq ($(PACKAGE_RPI_FIRMWARE_CONFIG_FILE),)
+ifneq ($(RPI_FIRMWARE_CONFIG_FILE),)
 define RPI_FIRMWARE_INSTALL_CONFIG
-	$(INSTALL) -D -m 0644 $(PACKAGE_RPI_FIRMWARE_CONFIG_FILE) \
+	$(INSTALL) -D -m 0644 $(RPI_FIRMWARE_CONFIG_FILE) \
 		$(BINARIES_DIR)/rpi-firmware/config.txt
 endef
 endif