linker: add CONFIG_KERNEL_LINK_IN_VIRT

This adds the kconfig CONFIG_KERNEL_LINK_IN_VIRT and supporting
kconfigs to indicate that the linker script can link the kernel
in virtual address space. This is needed for demand paging where
the kernel can be bigger than physical address space.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>