scripts/dts: Rename conf/header-writing functions to write_conf/header
scripts/dts: Rename conf/header-writing functions to write_conf/header

Probably easier to understand.

Also rename 'fd' to 'f'. "File descriptor" might make it sound like an
integer.

Signed-off-by: Ulf Magnusson <01743f1c1deb1fe75271d62edb65757d98ac0bcb@nordicsemi.no>

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