build: use find -perm /mode, instead of -perm +mode STRIP_FIND_CMD

The -perm +mode is deprecated, and sometimes yields suprising results.
It can be confused with permission in symbolic mode, for example '+u+g',
as POSIX spec suggests.

Signed-off-by: Tzu-Jung Lee <tjlee@ambarella.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>