west: build: Configurable build folder format

Add the possibility of configuring the build folder format in west's
configuration system.
The build.dir-fmt configuration option controls how west will create
build folders when not specified, the following parameters are currently
accepted:

- board: The board name
- source_dir: The relative path from CWD to the source directory
- app: The name of the source directory

If CWD is below source_dir in the directory hierarchy then source_dir is
set to an empty string.

This means that if one sets:

[build]
dir-fmt = build/{board}/{source_dir}

Then when building samples/hello_world from zephyr's root for the
reel_board the build folder will be:

./build/reel_board/samples/hello_world

but when building it from inside the samples/hello_world folder it will
instead be:

./build/reel_board

Fixes https://github.com/zephyrproject-rtos/west/issues/124

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>
Signed-off-by: Marti Bolivar <marti.bolivar@nordicsemi.no>
4 files changed