kernel: mm: rename Z_BOOT_* to K_MEM_BOOT_*

Rename Z_BOOT_VIRT_TO_PHYS() and Z_BOOT_PHYS_TO_VIRT() to
K_MEM_BOOT_VIRT_TO_PHYS() and K_MEM_BOOT_PHYS_TO_VIRT()
respectively. This is part of a series to move memory management
functions away from the Z_ namespace and into its own namespace.

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