[Skiboot] Boot tests gcov coverage reports

[Stewart Smith]

Hi all,

So, I just pushed some makefile/shell awfulness (feel free to fix it,
good luck though, I was about to go write LD_PRELOAD for GCC to make
this work properly) to be able to automagically produce code coverage
reports for the boot tests in mambo (POWER8 functional simulator).

basically, a build with SKIBOOT_GCOV=1 is nedeed, as is a zImage.epapr
for the boot test, then "make boot-coverage-report" will generate this:

http://open-power.github.io/skiboot/boot-coverage-report/

Which is distinct from the unit test code coverage report:

http://open-power.github.io/skiboot/coverage-report/


Open TODO items are:
- run these boot tests on hardware and extract GCOV report from hardware
  - combine runs on hardware with simulator for total code coverage
  report

- decipher the madness that is gcc and gcno/gcda with HOST and CROSS
  compilers (especially when they're not exactly the same gcc version)
  when combined with lcov (good luck)

- write some tests to test more of the code than a measly 20-30% of core/