rpi-userland: fix github tarball to use commit id

Signed-off-by: Spenser Gilliland <spenser@gillilanding.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

package/rpi-userland/rpi-userland.mk

@@ -5,7 +5,7 @@
 ################################################################################
 
 RPI_USERLAND_VERSION = 5e9a740a88a889dfc8a18bb1b00c17e5dd9d0108
-RPI_USERLAND_SITE = http://github.com/raspberrypi/userland/tarball/master
+RPI_USERLAND_SITE = http://github.com/raspberrypi/userland/tarball/$(RPI_USERLAND_VERSION)
 RPI_USERLAND_LICENSE = BSD-3c
 RPI_USERLAND_LICENSE_FILES = LICENCE
 RPI_USERLAND_INSTALL_STAGING = YES