doc: some doxygen cleanups

* removed RIOT unrelated cpu documentation
* introduced cpu_specific prefix to mark such documentation
* put oneway_malloc and crypto into sys group
doc: updated doxygen configuration

Updated with `doxygen -u`
doc: fix documentation for hwtimer

@verbatim was used incorrectly and a closing html tag was missing
doc: fixed remaining doxygen warnings in core

Only one warning remains and will be fixed in a separate commit.
doc: exclude more CPU specific stuff from doxygen
doc: fix doxygen warnings for color module

Also fixes the module name in API documentation.
doc: fix ambiguous file name warnings

doxygen does a good job itself, identifying the unique part of the path
to identify a file.
doc: fix naming for Cortex M3 common group
doc: put atmega cpu file into the right group
doc: fix vtimer documentation
doc: rename iot-lab_M3 group name
doc: add names to pca1000x boards
doc: improve doxygen cache size
doc: beautify HTML documentation

* add logo
* add description
* use RIOT specific color scheme
* add search functionality
* remove broken custom header, footer, and style sheet
doc: remove more CPU specific documentation
doc: fix warning from avr libc
Merge pull request #1844 from OlegHahm/doxygen_cleanup

Doxygen cleanup