Add a mechanism to construct a full cross-compiler environment using the
"crosstool-ng" cross-compiler building utility. This is implemented in
the new "common.crosstool" include, and augments the "dockcross/base"
base image.
Update Makefile to consolidate Dockerfile generation, notably the
"sed"-based inclusion directives.
Finally, employ all of this to generate a "linux-mips" 32-bit hard-float
MIPS cross-compiler Dockcross image.
