Added KERNEL_MASTER config flag
Added KERNEL_MASTER config flag

This makes it possible to statically check if the kernel is the master
kernel from user level. This is useful when when writing user level code
that's expected to work with different kernel versions.

cmr/seL4,zhicheng/seL4,cmr/seL4,cmr/seL4,zhicheng/seL4,zhicheng/seL4