kbuild: make 'archprepare' depend on 'scripts'

Before start descending, Kbuild needs to run 'prepare' and 'scripts',
which has been orthogonal to each other.

Going forward, let's consider 'scripts' is a part of the preparation.
This will allow more cleanups.

Move 'scripts' to the prerequisite of 'archprepare', where UML starts
compiling target *.c files.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>