cores: No standard include paths

cores: No standard include paths

There are problems with standard includes since the paths are ambiguous
between host and AVR. Using absolute includes does not work for multi-platform
projects.
The solution is to remove all standard includes of the host system for the
core files since there is no reason why a core definition should require
standard includes from the host system. User interaction (status messages)
is the job of the simulator and not the core definitions.

- add CORE_CFLAGS (-nostdinc) to Makefile.common
- remove <stdio.h> from core definition files
- move init message from core definition files to avr_init() in sim_avr.c

Signed-off-by: Stephan Veigl <veigl@gmx.net>