autotools.bbclass: mkdir ${B} -> mkdir -p ${B}

autotools.bbclass: mkdir ${B} -> mkdir -p ${B}

${B} is the default cwd of tasks, so there might be race issues such as:
| mkdir: cannot create directory `${B}': File exists
[snip]
NOTE: recipe perf-1.0-r9: task do_configure: Failed

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>