Allow paths of .rc files to be specified at mount_all

In current implementation, the mount_all command imports all .rc files
under /{system,vendor,odm}/etc/init/ after mouting filesystems is complete.
There's a need from ODMs to import different .rc files under different boot
modes (e.g., factory mode). Without this support, they will have to fiddle
around the init implementation.

This commit makes mount_all import the .rc files/directories specified
as additional arguments. If no path is given, the original ones are applied
the same way as the current implementation.

BUG: 26549689
Change-Id: Ie67ce13dde4c440ff8bf534826bc392c882a433f