Make: Change debug optimisation level to fit within flash size

[tracernz]

Advantages:

• drops debug build flash size to ~91k (from ~126k, almost overflowing)
• only enables optimizations that don't interfere with debugging [1]

Disadvantages:

• no longer supports GCC < 4.8 (released end of 2013)

[1] https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html#index-Ofast-903

-Og Optimize debugging experience. -Og enables optimizations that do not interfere with debugging. It should be the optimization level of choice for the standard edit-compile-debug cycle, offering a reasonable level of optimization while maintaining fast compilation and a good debugging experience.

[ghost]

Can one of the admins verify this patch?

[trollcop]

ok to test