base-files: add support for platform_pre_upgrade function

Current NAND sysupgrade process is a bit hard to follow due to the way
of triggering stage1. Currently this is done by leaving a /mark/ in the
form of /tmp/sysupgrade-nand-path during nand_do_platform_check.
Existence of this mark stops standard sysupgrade process (as the result
of sysupgrade_pre_upgrade exit). This may be a bit misleading.

Proposed solution adds a new function that will allow platform.sh
trigger NAND sysupgrade consciously. This will also allow cleaning
nand_do_platform_check limiting it to just checking the image.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@45338 3c298f89-4303-0410-b956-a3cf2f4a3e73

package/base-files/files/sbin/sysupgrade

@@ -215,6 +215,14 @@ fi
 
 run_hooks "" $sysupgrade_pre_upgrade
 
+# Some platforms/devices may want different sysupgrade process, e.g. without
+# killing processes yet or calling ubus system upgrade method.
+# This is needed e.g. on NAND devices where we just want to trigger stage1 at
+# this point.
+if type 'platform_pre_upgrade' >/dev/null 2>/dev/null; then
+	platform_pre_upgrade "$ARGV"
+fi
+
 ubus call system upgrade
 touch /tmp/sysupgrade