net: shell: Add network interface up/down command
net: shell: Add network interface up/down command

User is able to take a network interface down or bring it up.
The command syntax is "net iface [up|down] [index]"

Signed-off-by: Jukka Rissanen <f9f077d1da4c7aa947f9864fa5a28c3588f6a45d@linux.intel.com>

galak/zephyr,zephyriot/zephyr,ldts/zephyr,mbolivar/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,nashif/zephyr,nashif/zephyr,galak/zephyr,punitvara/zephyr,zephyrproject-rtos/zephyr,zephyriot/zephyr,finikorg/zephyr,kraj/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,finikorg/zephyr,explora26/zephyr,nashif/zephyr,explora26/zephyr,zephyriot/zephyr,mbolivar/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,mbolivar/zephyr,explora26/zephyr,punitvara/zephyr,GiulianoFranchetto/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,kraj/zephyr,punitvara/zephyr,punitvara/zephyr,mbolivar/zephyr,kraj/zephyr,punitvara/zephyr,kraj/zephyr,galak/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,Vudentz/zephyr,ldts/zephyr,galak/zephyr,kraj/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,mbolivar/zephyr,explora26/zephyr,zephyriot/zephyr,ldts/zephyr,explora26/zephyr,zephyriot/zephyr,nashif/zephyr,finikorg/zephyr,Vudentz/zephyr,Vudentz/zephyr