net/l2: Switch OpenThread L2 to new net_pkt allocator
net/l2: Switch OpenThread L2 to new net_pkt allocator

Maybe there is an easier way to fill in the buffer in one call from an
otMessage.

Switch "frag" keyword to "buf", as "frag" is now reserved for actual
packet fragmentation and not buffer.

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

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