kernel: add kconfig to say not all code/data is present at boot

With demand paging and pinned sections enabled, it is possible for
data to be brought into physical memory as required by current
execution context. However, the kernel still assumes that all data
pages are present at boot which may not be desirable for certain
scenarios. This introduces a new kconfig to specify that those
data pages other than the boot and pinned sections are not present,
and they would be paged in on demand.

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