scripts: Refactor build-all.py's build

Build all consists of a mismash of figuring out variables, running
shutils, and actually executing commands.  In preparation for parallel
builds, change this to build up the commands in advance, and then have
a tracker go through and actually run the commands.

Change-Id: I24215b14e5d045f8269bbdc2af9989ab21b2310b
Signed-off-by: David Brown <davidb@codeaurora.org>