package.bbclass: Check FILES once, not once per loop iteration

package.bbclass: Check FILES once, not once per loop iteration

There is no need to check FILES in each loop iteration, we can just check it once
at the start when we read the variable.

(From OE-Core rev: 1bcc5cba12cbb1b846b433719d1b0820f5a97105)

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