utils: Reduce the number of calls to the "dirname" command

utils.bbclass contains create_cmdline_wrapper() function that
creates wrapper script with additional arguments for any passed
"$cmd" command, and uses several calls to "dirname".

Because "dirname" is an external command, in cases of lots of
calls to wrapped "$cmd", each call of "dirname" will incur
significant overhead.

There are three same calls to "dirname": one for saving it`s
output to "realdir" variable, and other two in "exec" command.
So last two "dirname" calls can be replaced with cached value
from "realdir" variable.

Signed-off-by: Oleksandr Popovych <opopovyc@cisco.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 4b9cf2c8)
Signed-off-by: Steve Sakoman <steve@sakoman.com>