SYSTEM_ROOT -> get_system_root

Add a get_system_root() function in roots.cpp
which returns / or /system based on fstab.
This factors out the 'if' check from recovery.cpp
and fsck_unshare_blocks.cpp.

Test: boot recovery, mount system
Bug: 118634720
Change-Id: If4947cba3a3f08dbe695fd2b50a9354cd302ee03