repack_bootimg: changes the default path of --ramdisk_add

Commit I55dc8ff322f6b97e2d6dc1a4ee5935e863f2f835 moves debug
ramdisk resources to the root of the ramdisk, instead of
/first_stage_ramdis.

Changing the default copy path from
'first_stage_ramdisk/userdebug_plat_sepolicy.cil' to
'/userdebug_plat_sepolicy.cil' in repack_bootimg.

Bug: 186082603
Test: repack_bootimg --src_bootimg boot-5.10-gz.img --dst_bootimg boot-debug.img
Change-Id: Id0ce67d4757ce4ce1bcd849e466e99965c0099e2