busybox: make postinst run firstly before update-alternatives

busybox: make postinst run firstly before update-alternatives

The update-alternatives.bbclass' postinst script runs firstly before other
postinst, but busybox needs set basic tools such as sed command firstly,
otherwise, update-alternatives doesn't work, so run busybox' postinst firstly
to fix the problem.

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>