post-build.sh: Support using @ as space in partition table

For example:
RK_EXTRA_PARTITIONS="oem@userdata:/media:vfat:errors=remount-ro:userdata_empty"
->
PARTLABEL=oem /oem auto defaults 0 2
PARTLABEL=userdata /media vfat errors=remount-ro 0 2

Change-Id: I54fec6c28c60cab347bb56ac5a2208d2a6d0e238
Signed-off-by: Jeffy Chen <jeffy.chen@rock-chips.com>

common/post-build.sh

@@ -56,7 +56,7 @@ function fixup_fstab()
 			;;
 	esac
 
-	for part in ${RK_EXTRA_PARTITIONS}; do
+	for part in ${RK_EXTRA_PARTITIONS//@/ }; do
 		fixup_part $(echo "${part}" | xargs -d':')
 	done
 }