cpu/cc2538: centralized all cpu specific includes

- added all CPU specific includes to cpu-conf.h
- removed now not needed includes from cpu .c files
cppcheck: handle --diff-filter option

Add the possibility to the cppcheck script to pass git's
`--diff-filter` option.
Also improve comments a bit.
Travis: let cppcheck fail for new files

Change Travis configuration for cppcheck to fail on new files and keep on
warning about existing ones.
cpu/cc2538: cleaned up IRQ vector definition
board/cc2538dk: made ISR functions configurable
cpu/cc2538: made ISR names configurable by a board
Merge pull request #1862 from haukepetersen/fix_cc2538_includecleanup

cpu/cc2538: centralized all cpu specific includes
Merge pull request #1861 from haukepetersen/fix_cc2538_irqvector

cpu/cc2538: cleaned up IRQ vector definition
Merge pull request #1840 from LudwigOrtmann/cppcheck-selecta

Travis + cppcheck: let cppcheck fail for new files