recipetool/devtool: calculate source paths relative to UNPACKDIR

recipetool/devtool: calculate source paths relative to UNPACKDIR

Now that recipes default to S in UNPACKDIR, recipetool and devtool should
do the same.

There was some discussion about changing devtool to simply setting
UNPACKDIR via bbappend to a workspace and running unpack task directly;
currently it has a bunch of convoluted path calculations, substitutions,
moving source trees around and and special casing (devtool-source.bbclass
in particular is an unpleasant hack).

This should definitely be done; but right now we can simply tweak existing
code which at least doesn't make it worse.

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>