add SNMP readout support for DHT11 sensors

Add SNMP handlers for reading out humidity and temperature values
provided by DHT11 sensors.

Currently, ethersex is limited to only one such sensor. A future
extension to multiple sensors would be possible, the SNMP OID layout of
this implementation has been chosen with that possibility in mind.

SNMP OIDs are
SNMP_OID_ETHERSEX.5.1.n for the polling delay of sensor no. n [HZ]
SNMP_OID_ETHERSEX.5.2.n temperature sensor no. n [deg Celsius * 10]
SNMP_OID_ETHERSEX.5.3.n rel. humidity of sensor no. n [% * 10]

Only n=0 is implemented due to the constraints outlined above.
Fix ordering of config menu
add help text to DHT_SNMP_SUPPORT menuconfig
simplify menuconfig heading
Merge pull request #268 from arwarw/feature-dht11_snmp_support

add SNMP readout support for DHT11 sensors
Changed shell to bash, so it works on Mac OS X too.
Merge pull request #269 from joerghermann/master

Changed shell to bash, so it works on Mac OS X too.
Only trailing spaces removed.
Added USART selection for serial debug with USART 0 as default.
Don't consume USART when debug is routed to Syslog.
Correct USART counting for combinations of ECMD, Syslog and Debug.
Added missing modules requesting USART, ordered the list alphabetically.
Removed hardcoded debug on USART 1, selection is now available from the debug menu.
added beteigeuze profile
Merge pull request #272 from sauttefk/e6-upstream4

added beteigeuze profile
Merge branch 'USART' of github.com:joerghermann/ethersex into joerghermann-USART
preselect usart1 for debugging
Merge pull request #274 from eku/joerghermann-USART

Usart channel selection for serial line debugging
YPORT: retransmit the identical data on uip_rexmit()
fix typo in watchasync