dts: Rename generated_dts_board*.{h,conf} to devicetree*.{h,conf}

generated_dts_board.h is pretty redundant and confusing as a name. Call
it devicetree.h instead.

dts.h would be another option, but DTS stands for "devicetree source"
and is the source code format, so it's a bit confusing too.

The replacement was done by grepping for 'generated_dts_board' and
'GENERATED_DTS_BOARD'.

Two build diagram and input-output SVG files were updated as well, along
with misc. documentation.

hal_ti, mcuboot, and ci-tools updates are included too, in the west.yml
update.

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