scripts: fix default work-dir name

It's been a long time the default work-dir changed its name
from 'target' to '.build'.

Change the left-over.

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

CT_EXPERIMENTAL=y

CT_LOCAL_TARBALLS_DIR="${HOME}/src"

CT_SAVE_TARBALLS=y

CT_CONFIG_SHELL_SH=y

CT_LOG_EXTRA=y

CT_ARCH_avr32=y

CT_BINUTILS_V_2_18a=y

CT_CC_V_4_2_2=y

CT_LIBC_NEWLIB_V_1_17_0=y