Make kernel prebuilts writable after copying them.

Images created by Bazel are read-only, which leads to build failures when
later commands attempt to modify them, as the cp command will also copy
the file permissions. Fix it by using chmod to make the images writable
after copying them.

Change-Id: I8aaa7c4395d38b674f76a7f0ebfa95e1f2da0af0