bb.utils: check if lock file is writable, to fix bug 606
Bug 606 report that if $DL_DIR is read-only, do_fetch will
simply hang without any error message.
The root cause is that: bb.fetch.go()->bb.utils.lockfile()
will try to lock file ${DL_DIR}/xxxxx.lock. Since ${DL_DIR}
is read-only, it will cause IOError exception. Although
lockfile() can catch the exception, currently code simply
ignore all the exception and continue the loop. it make
sense if the exception is caused by locking contention,
but in the read-only $DL_DIR case, it cause endless waiting
unfortunately.
So this patch add read-only check for lockfile to avoid the
silent hang.
