return to using subprocess for running commands

Reverts Ic4f1c747 and fixes the problem by splitting the extra_flags
argument (it wasn't working before because it was getting one argument
"-c 4096 -s 128" instead of four arguments "-c", "4096", "-s", "128"
and mkyaffs2image apparently just ignores that bogus argument?)

Change-Id: Ib1e08d634aa68bfab0f7e09680d407f2cee0797d