C-API example: fix runtime issues

[enrikb]

• Linker script is no longer generated, but libopencm3 uses the fixed configuration style.

• The ccm RAM region leftover from the STM32F4 configuration has been removed. Now, end is correctly calculated by the linker and libc actually works, no longer overwriting global variables.

• rules.mk refreshed from libopencm3-template and linker map generation enabled.

• Example checks verify return value, prints it out and also returns the result from main(). Actually, _exit() is used to make QEMU return, too.

This can now be run like so:

$ make run-example
# Get out via Ctrl-A X in case QEMU does not return
qemu-system-arm -cpu cortex-m33 -machine musca-b1 -nographic \
  -semihosting-config enable=on,target=native -kernel example/example.elf
signature generated, verify returns: 3
make: *** [Makefile:25: run-example] Error 3

(The original finding from #4 has intentionally not been fixed, to show the testing possibility like above. Otherwise, this fixes #4.)

[nickray]

This is so cool, I'm terribly thankful :)