This commit uses the previously introduced feature of generating
configuration data as a C header file rather than as command line macro
definitions. Each toolchain was modified to use prefix headers if
requested, and build_api.py was modified to set up the toolchain's
prefix header content using the data generated by the config system.
Tested by compiling blinky for GCC and ARMCC. I'm having a few issues
with my IAR license currently, but both ARMCC and IAR use the same
`--preinclude` option for prefix headers, so this shouldn't be an issue.
Note that at the moment all exporters still use the previous
configuration data mechanism (individual macro definitions as opposed to
a prefix header). Exporters will be updated in one or more PRs that will
follow.
Bogdan Marinescu
Jimmy Brisson
Martin Kojtal
Vincenzo Frascino
Mihail Stoyanov
Russ Butler