add patch generation scripts

With configure.sh removed in accfbe8c03854fdda2903f0486ed66efdb904df9, this
repo contained patch files, but did not provide any way to create them.

This commit adds scripts that dual as patch generator (for release ebuilds)
and live patcher (e.g for a 9999 ebuild):

* bin/tlp-editsrc.bash is a helper for editing source files
  Roughly what configure.sh did, but less "usage-friendly" (does not accept
  many options etc.) and more feature-loaded.
  It accepts a series of commands as input ("CFGVAR TLP_ENABLE 0", "MACRO
  no-radiosw", ...) and applies corresponding sed commands to the source dir.

  Already incorporates some of the "hacks" for buildroot packaging.

* the Makefile is a wrapper on top of it that handles both live-patching
  ("livepatch-{base[patch],no-deprecated,unbundle-tpacpi-bat" targets)
  and patch file generation + upload ("genpatches" target) for the
  gentoo ebuild

To summarize the changes implied by this commit,
this repo has 3 use cases now:
* gentoo    : patch generation for tlp upsteam release
* gentoo    : livepatch for -9999 ebuild
* buildroot : "livepatch-basepatch" + additional files (not migrated yet)
import devlist-sh from buildroot-packages
import files from buildroot-packages