package.bbclass: Add a shortcut exit from the mkdir function

package.bbclass: Add a shortcut exit from the mkdir function

The mkdir function iterates over strings with many different operations,
even if ultimately the target already exists. This adds a check to the start
of the function so we don't waste time when the target already exists.

(From OE-Core rev: 06e188e673313f1eb9ae7d85ae64467c8d2a94b3)

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