Introduce a new Makefile holding the common functionality for building tools
used by Rockbox Utility (*patcher / mk*boot). This converts mktccboot to use
the common Makefile.
Also introduces BUILD_DIR variable to control the path used for placing
intermediate objects into. This should avoid filename clashes between different
tools. Filenames are (still) assumed to be unique withing a single tool.
git-svn-id: svn://svn.rockbox.org/rockbox/trunk@31257 a1c6a512-1295-4272-9138-f99709370657
