stm32: Move system stub to nosys library

This solves problems with missing symbols like _exit. When nosys.spec is
mentioned on gcc command line, libnosys is automatically linked with the
program the right way (i.e. in group with -lc).

Also, due to this, stm32rt library was renamed to stm32, because it no
longer contains "run time" support.
Simplify demos Makefile
Simplify makefiles for individual demos

Moving stm32 platform support to separate libraries allows us to
simplify the Makefiles. Now the makefiles do not contain platform
depended parts and all "ifeq/endif" blocks can be removed.
Move linux-specific program to target/linux subdirectory

Also ts.c and ks.c files, which are platform independent, are now
unconditionally compiled into the macan library.
Simplify macan library Makefile
Move target-specific files one directory up

This simplifies directory structure and makefiles.
stm32: Symlink headers instead of copying
linux: Fix Makefile