config: change name of default build dir

'targets' is not really meaningfull.
'build' means what it means.
'.build' just hides it as well! :-)

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 reporter_name="Yann E. MORIN"

 reporter_url="http://ymorin.is-a-geek.org/dokuwiki/projects/crosstool"

 reporter_comment=""