arch, vmm: Start documenting major regions of RAM and reserved memory

Using the existing layout module start documenting the major regions of
RAM and those areas that are reserved. Some of the constants have also
been renamed to be more consistent and some functions that returned
constant variables have been replaced.

Future commits will move more constants into this file to make it the
canonical source of information about the memory layout.

Signed-off-by: Rob Bradford <robert.bradford@intel.com>

vmm/src/vm.rs

@@ -687,7 +687,7 @@ impl Vm {
             mem.deref(),
             None,
             &mut self.kernel,
-            Some(arch::HIMEM_START),
+            Some(arch::layout::HIGH_RAM_START),
         ) {
             Ok(entry_addr) => entry_addr,
             Err(linux_loader::loader::Error::InvalidElfMagicNumber) => {
@@ -695,7 +695,7 @@ impl Vm {
                     mem.deref(),
                     None,
                     &mut self.kernel,
-                    Some(arch::HIMEM_START),
+                    Some(arch::layout::HIGH_RAM_START),
                 )
                 .map_err(Error::KernelLoad)?
             }