Boot: update deprecated comments.

Updates comments on boot requirements:
- the GDT is a simple identity table
- the boot fails in crt0 if stage1 returns.

Testing done: build & qemu -kernel out/kernel/Kernel/Kernel.bin
Utils: reintroduce tests.

Restores Utils library tests and fix some compile errors in them.

Testing done: make kernel
[...]
TEST  Utils
Running 13 tests.............OK!
[...]
Build: protect test generator from empty input.

Ensures that the user declared test header files in LOCAL_TESTS_FILES in build
specifications to avoid crashing CxxTest code generator.

Testing done: remove header files from Utils' LOCAL_TESTS_FILES and check that
an error appears when building:
$ make kernel
build/parse-module.mk:96: *** Utils: missing test header files in LOCAL_TESTS_FILES.  Stop
Then restore the variable and build again, everything is OK.