ipq806x: base-files: asrock: fix bootcount include

Fixes following warning message during image building process:

 Finalizing root filesystem...
 root-ipq806x/lib/upgrade/asrock.sh: line 1: /lib/functions.sh: No such file or directory
 Enabling boot
 root-ipq806x/lib/upgrade/asrock.sh: line 1: /lib/functions.sh: No such file or directory
 Enabling bootcount

Fixes #9350

Fixes: 98b86296 ("ipq806x: add support for ASRock G10")
Signed-off-by: Petr Štetiar <ynezz@true.cz>

target/linux/ipq806x/base-files/lib/upgrade/asrock.sh

-. /lib/functions.sh
+. "$IPKG_INSTROOT/lib/functions.sh"
 
 asrock_bootconfig_mangle() {
 	local mtdnum="$(find_mtd_index 0:bootconfig)"