staging: Reduce the number of mkdirs calls

staging: Reduce the number of mkdirs calls

The number of mkdir calls was showing up high on the profile charts since
it was getting called once per file which is excessive. Each call results
in one or more syscalls which is bad for performance. Cache which
directories we've seen to reduce the calls to a more reasonable number
and speed up recipe specific sysroots.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>