Simplify Debian packaging for PRU GCC toolchain

[dinuxbg]

Current debian packaging rules on https://github.com/beagleboard/repos are derived from Debian's avr-gcc. The rules are rather complicated, and result in clashes (e.g. you can install either pru-gcc or avr-gcc, but not both).

pru-gcc could be packaged more easily if it is installed into a separate directory, and only create symbolic links for /usr/bin/pru-gcc and the rest of the tools' executables. See some initial ideas here: https://github.com/dinuxbg/repos/tree/wip-revamp-debian-rules

Unlike issue #20 which has an ambition to include pru-gcc into official Debian, this issue is smaller in scope and should be easier to implement.