kernel: add kconfig CONFIG_KERNEL_MEM_POOL

Adds a kconfig CONFIG_KERNEL_MEM_POOL to decide whether
kernel memory pool related code is compiled. This option
can be disabled to shrink code size. If k_heap is not
being used at all, kheap.c will also not be compiled,
resulting in further smaller code size.

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