build: fix find warning with SCAN_EXTRA

If you change SCAN_EXTRA variable with "-path target/linux/xxxx" in
include/toplevel.mk for speed up scan, find will warn with:

find: warning: you have specified the global option -maxdepth after
the argument -path, but global options are not positional, i.e.,
-maxdepth affects tests specified before it as well as those specified
after it.  Please specify global options before other arguments.

The find option -mindepth -maxdepth are global options and must be
before any path option. Change order of $(SCAN_EXTRA) after -mindepth
and -maxdepth to fix this.

Signed-off-by: Leo Chung <gewalalb@gmail.com>
[capitalize Description, Author and Sob and minor description tweak]
Signed-off-by: Christian 'Ansuel' Marangi <ansuelsmth@gmail.com>