linux: allow both in-tree and custom dts files

For some boards, for example the Raspberry Pi, it's necessary to build
in-tree dts files as well as custom/out of tree dts-files (dt-blob.bin).
The existing logic made these two options exclusive, this commit changes
that to allow both in-tree as well as custom sources for dts files.

Signed-off-by: Simon van der Veldt <simon.vanderveldt@gmail.com>
[Arnout: re-wrap help, add extra empty line, change = into +=]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>