Ignore error with uClibc cleaning when builds linux-uclibc toolchain.

2013-03-27  Anton Kolesov <anton.kolesov@synopsys.com>

    * build-uclibc.sh: Ignore error which happens when running 'make
    distclean' on uClibc. make will fail if there is no .config file already
    present, but this file is generated only after distclean by defconfig.