scons: interpret paths relative to launch directory

scons: interpret paths relative to launch directory

Make sure all command-line targets and EXTRAS directories
are interpreted relative to the launch directory.  This
turns out to be very useful when building code from an
EXTRAS directory using SCons's -C option.

We were trying to do this with targets but it didn't actually
work since we didn't update BUILD_TARGETS (so SCons got
confused internally).  We weren't even trying with EXTRAS.

To simplify the code, the default target is also interpreted
relative to the launch dir even though it was explicitly
handled as relative to the m5 dir before... I doubt anyone
really uses this anyway so it didn't seem worth the complexity.
(Maybe we should get rid of it?)