scripts/dts: Simplify output_keyvalue_lines()
scripts/dts: Simplify output_keyvalue_lines()

That the 'aliases' key isn't a regular property makes things a bit
confusing. Write the aliases separately, which should be a bit clearer.
That also makes it clear that the order doesn't matter.

Also write "'aliases'" instead of "prop" when the value is known.

The .get() seems to be needed to work around a bug, and sometimes you
get DT_...=None in the output. Might look into that later.

extract_string_prop() could generate 'defs' entries without 'aliases'
keys. Make it add an empty 'aliases' dict instead, to remove existence
checks elsewhere.

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

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