minios: introduce minios_get_l1prot

Signed-off-by: Wei Liu <liuw@liuw.name>

Provide page allocator as part of bmk-core.

Platforms now share common page allocator code.

make bmk_memsize a variable instead of a function call

Since the page mallocator returns va, make it return void *

Remove all the silly casts from calls to bmk_pgalloc

convert xen platform to use the bmk_pgalloc() interface

remove sporadic ifdefs for the not-supported ia64 arch

Add bmk_platform_memsize() and replace use of homegrown variants.

convert platform/xen to rumprun_core memalloc

Separate bmk-common to bmk-base and bmk-core, mirroring the libs

This mechanical change makes it easy to look at the headers and determine
at which layer a file is, instead of having to wonder if some bmk-common
interface is provided by -base or -core.

sensible layering drama, act 2

Convert easy cases of xen platform string routine use from libc to
bmk-string. There are a few harder-to-crack ones which use things
like strtotul. Need to see if those are really necessary, or can we do
without introducing those interfaces to the desirably minimal bmk-string.

Move files into platform/xen

Prepares for the rumprun merge.