fs_mgr: Add support for A/B partitions

Allow partitions to be marked as A/B partitions
using the slotselect flag in fstab. The partitions
can be identified by appending the correct suffix
to the block device listed in the fstab. The suffix
is provided by the bootloader through a command line
parameter or the device tree, and can be found in
ro.boot.slot_suffix or read from the boot_control HAL.

Change-Id: I6846d80e857f95bfb8f282f4ab81167394613bbe
Signed-off-by: Daniel Rosenberg <drosen@google.com>