This commit refactors USB string code, making it, hopefully, less
buggy and more easier to understand. It also removes "magic" bit
manipulation and "magic" numbers;
All #includes now explicitly use the "<libopencm3/stm32/rcc.h>" format.
If you want to get rid of the "libopencm3" prefix in your local project you
can add a respective -I entry in your Makefile (not recommended though).
All .ld files and .a libs are installed in $(TOOLCHAIN_DIR)/lib
directly (as before), but are now renamed to avoid potential
conflicts now or in the future. Examples:
libopencm3_lpc13xx.a
libopencm3_lpc13xx.ld
libopencm3_stm32.a
libopencm3_stm32.ld
