net/pkt: Introducing a new helper to get the start of the buffer
net/pkt: Introducing a new helper to get the start of the buffer

This is mostly meant to be used in drivers and/or L2.

net_pkt_ll() is going to be removed and is not semantically right.
Also, net_pkt_ip_data() is not semantically right as it is meant to
access IP data start.

So instead, adding a new function: net_pkt_data() which will get the
start of the buffer.

Signed-off-by: Tomasz Bursztyka <ba81a3a719836727e6857ae83462b9f52ec41006@linux.intel.com>

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