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>

arch/um/drivers/mconsole.h

@@ -7,7 +7,7 @@
 #ifndef __MCONSOLE_H__
 #define __MCONSOLE_H__
 
-#ifndef __KERNEL__
+#ifdef __UM_HOST__
 #include <stdint.h>
 #define u32 uint32_t
 #endif