um: Stop abusing __KERNEL__

Currently UML is abusing __KERNEL__ to distinguish between
kernel and host code (os-Linux). It is better to use a custom
define such that existing users of __KERNEL__ don't get confused.

Signed-off-by: Richard Weinberger <richard@nod.at>

diff --cc arch/um/Makefile
Simple merge

diff --cc arch/um/drivers/mconsole.h
Simple merge

diff --cc arch/um/include/shared/init.h
Simple merge

diff --cc arch/um/include/shared/user.h
Simple merge

diff --cc arch/x86/um/shared/sysdep/tls.h
Simple merge