merge of SIP components from EtherGraf's
git@github.com:EtherGraf/ethersex.git sip
dropping doorbell files
help text for SIP_SUPPORT
reformatted source code to comply Ethersex's style guide
no more avrdude rules in common Makefile
finally removed all left-overs
replaced strcpy/strcat with sprintf
use prefix 'sip_' for static variables to avoid conflicts with other modules
Merge branch 'EtherGraf/sip'

closes #252

Signed-off-by: Maximilian Güntner <maximilian.guentner@gmail.com>