build.sh: fixed -a build, renamed to -o
build.sh: omit benchmarks build by default

To be friendlier to new users of Patmos, build.sh does not build bench by
default (takes forever):
 * Developers are considered smart enough to create a build.cfg and change this
   behavior if required.

Finally, the -a (all) option now really does what you would expect and builds
$ALLTARGETS.
Merge pull request #5 from alexjordan/user-friendly

build.sh: user friendlier default behavior