Building any application with gcc 7.1.1 fails with the following error:
```
~/RIOT % make -C examples/hello-world
make: Entering directory 'RIOT/examples/hello-world'
Building application "hello-world" for "native" with MCU "native".
"make" -C RIOT/boards/native
"make" -C RIOT/boards/native/drivers
"make" -C RIOT/core
"make" -C RIOT/cpu/native
RIOT/cpu/native/startup.c: In function ‘startup’:
RIOT/cpu/native/startup.c:292:17: error: this statement may fall through [-Werror=implicit-fallthrough=]
usage_exit(EXIT_SUCCESS);
^~~~~~~~~~~~~~~~~~~~~~~~
RIOT/cpu/native/startup.c:293:13: note: here
case 'i':
^~~~
cc1: all warnings being treated as errors
make[2]: *** [RIOT/Makefile.base:81: RIOT/examples/hello-world/bin/native/cpu/startup.o] Error 1
make[1]: *** [RIOT/Makefile.base:20: ALL--RIOT/cpu/native] Error 2
make: *** [RIOT/examples/hello-world/../../Makefile.include:297: all] Error 2
make: Leaving directory 'RIOT/examples/hello-world'
```
```
% gcc --version
gcc (GCC) 7.1.1 20170528
Copyright (C) 2017 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
```
fixed by adding a `break` statement after `usage_exit()`
The new implementation uses a precalculated map of which mode to switch
to next if going from mode A to mode B. This simplifies the
implementation for moving between modes which are not direct neighbors.
See mcg.h documentation for a diagram of the state machine for the
clocking modes. Also found in the CPU reference manual of all Kinetis
CPUs, MCG chapter, MCG mode state diagram.
The use of symbolic links leads to trouble in certain environments
(e.g. vagrant under Win). This PR gets rid of symlinks and uses
variables in the Makefiles to map to the correct linkerscripts
instead.
- removed ISR_STACKSIZE define where unused (set to 0)
- removed thread_arch_isr_stack_usage(), thread_arch_isr_stack_start(),
and/or thread_arch_isr_stack_pointer() where not implemented
