The CPU variable in the boards Makefile.include file already contains the target
CPU, so there is no reason to provide it in each board again as avrdude flag.
This commit automatically sets the avrdude target from the CPU variable and
removes the unneeded flags.
Currently the flag "-P ${PORT}" is added to avrdude regardless of the programmer
used. But this flag should only be set for programmers that operate over a
serial port - e.g. like the various Arduino bootloaders. This commit changes
the behaviour so that the "-P flag" is only set for only of the default
programmers of the various AVR boards supported by RIOT. This allows to use
ICSP programmers (e.g. like the usbtiny) like this:
make BOARD=arduino-uno PROGRAMMER=usbtiny
Allows to use avrdude as a flashing tool in any context
(e.g. not dependent on arduino or atmega) though it only
works (AFAIK) on atmega, but I thought it's better to
have it here as we have other flashing tools.
