Semi-automatic support of new devices

[rwirth]

I am trying to build an avr-libc that supports new devices like the mega 4809. While the auto-generated IO headers are in place, there are (at least) some additional hurdles to overcome:

• The device-specific directories below avr/lib are not generated by gen-avr-lib-tree because the list of devices is hard-coded.
• Same thing for configure.ac, where each of the generated Makefile.am has to be added.
• The iosym files for the new devices are missing, causing the CRT build to fail.

I think I have solved the first two problems by having gen-avr-lib-tree parse the spec-files used by the installed avr-gcc. The parse generates the same data structure that was used before. To tie the generated files into autotools gen-avr-lib-tree also generates an M4 file with an AC_CONFIG_FILES macro containing all generated files. That file can be included by configure.ac.

The missing iosym files could be added as stubs for now, they should probably be generated from the atdf files if I understand the process correctly.

I have created a branch in my fork that contains the changes.

Is this useful or are there issues that I have overlooked?