Avoid adding an extra slash if OUT_DIR_COMMON_BASE ends in a slash

The extra slash can create all kinds of problems throughout the
makefiles.  In particular, it caused a device to become unbootable
because the ramdisk was incomplete due to mismatched filenames
(some containing / and some containing //).

Change-Id: I9dbb425aaf51e7ed2697dba0034f2ee92ab3c82b