kernel-devsrc: remove python2 dependency

kernel-devsrc: remove python2 dependency

Witht the approaching EOL of python2, the kernel packages need to
be updated to depend on python3.

The core kernel scripts are now python3 safe, making the RDEPENDS
change relatively simple.

but

There are some scripts that are captured in the devsrc files that
are not used, but reference /usr/bin/python. The python3 package
does not provide /usr/bin/python so any package managers with
automatic dependency detection will fail to assemble a rootfs due
to the missing intepreter.

We could drop the scripts from the source capture, but that
risks dropping something that we need in certain operations and
we'd miss when they are updated in tree to explicitly call python3
(since they won't be revisited often). Light testing shows that the
scripts do run with python3, so rather than removing them, we change
the interpreter once they are copied, and automatic dependency
detection won't cause us rootfs assembly issues.

(From OE-Core rev: 6745088b6c9e5c70df59d667347ede470df9a91d)

Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>