cmake: ensure consistent behavior between x_ROOT settings

Fixes: #23825

Today, BOARD, SOC, ARCH, DTS, KCONFIG, and TOOLCHAIN_ROOT can be
specified by users or through other CMake files.

It is not clear if relative paths are permitted and/or from where a
relative path is constructed.

Inside CMake, it is very easy to specify `${ZEPHYR_BASE}`,
`${CMAKE_CURRENT_SOURCE_DIR}`, or similar, thus there is no reason to
use relative path inside CMake, as it easy can lead to discussion on
relative to what.

For users manually invoking CMake using, `cmake ... <app-dir>` it is
nice to have to possibility to specify a relative path.
For this purpose, relative path are considered relative to the
`APPLICATION_SOURCE_DIR`.

This commit updates:
- BOARD_ROOT
- SOC_ROOT
- ARCH_ROOT
- DTS_ROOT
- KCONFIG_ROOT
- TOOLCHAIN_ROOT

to allow paths relative to `APPLICATION_SOURCE_DIR` when specified with
`-D<type>_ROOT=<relative-path>`

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>