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
fix e6 issue 253 - new script osdefaults.sh to be sourced to get default command for OS
YPORT: retransmit the identical data on uip_rexmit()
fix typo in watchasync
fix typo in pinning/hardware/antares_O.m4
YPORT: handle uip_rexmit() properly
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>
Merge pull request #254 from michaelb42/e6_osdefaults

fix e6 issue 253 - new script osdefaults.sh to be sourced to get default...
grep/cut not needed if sed is used
loop replaced with sed one-liner
replace perl with awk
define usart number for modbus/eltakom
splitted up Makefiles into subdirectories

- custom modules can now be included in one of the
  custom.mk files

Signed-off-by: Maximilian Güntner <maximilian.guentner@gmail.com>
added custom.mk to .gitignore

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