package/rpi-firmware: fix BR2_PACKAGE_RPI_FIRMWARE_VARIANT_PI_X

Fix the following build failure raised since the rework of boot/config
file handling in commit 689b9ac439ab7b507c8982b6102bddf59d03efbf:

/usr/bin/install: cannot stat '/home/giuliobenetti/autobuild/run/instance-3/output-1/build/rpi-firmware-2ef3800b1b71c0e30c2a61ecad6e007ae96ea342/boot/startx.elf': No such file or directory

Fixes:
 - http://autobuild.buildroot.org/results/88b2137ea81c4944cbded11fa68adcbca02e83d0

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Reviewed-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

package/rpi-firmware/rpi-firmware.mk

@@ -13,7 +13,7 @@ RPI_FIRMWARE_INSTALL_IMAGES = YES
 RPI_FIRMWARE_FILES = \
 	$(if $(BR2_PACKAGE_RPI_FIRMWARE_BOOTCODE_BIN), bootcode.bin) \
 	$(if $(BR2_PACKAGE_RPI_FIRMWARE_VARIANT_PI), start.elf fixup.dat) \
-	$(if $(BR2_PACKAGE_RPI_FIRMWARE_VARIANT_PI_X), startx.elf fixupx.dat) \
+	$(if $(BR2_PACKAGE_RPI_FIRMWARE_VARIANT_PI_X), start_x.elf fixupx.dat) \
 	$(if $(BR2_PACKAGE_RPI_FIRMWARE_VARIANT_PI_CD), start_cd.elf fixup_cd.dat) \
 	$(if $(BR2_PACKAGE_RPI_FIRMWARE_VARIANT_PI_DB), start_db.elf fixup_db.dat) \
 	$(if $(BR2_PACKAGE_RPI_FIRMWARE_VARIANT_PI4), start4.elf fixup4.dat) \