hw/x86: Load configuration from multiboot module if found

Retire bmk_page{size,shift}, use BMK_PCPU_PAGE_{SIZE,SHIFT}.

Latter are compile-time constants.

Allow libbmk* to include platform/cpu-specific headers.

Use the <bmk-pcpu> namespace for that (pcpu = platform+cpu).

The purpose is to allow some compile-time and other optimizations to
take place, e.g. with PAGE_SIZE.

actually commit the code changes too (for previous commit)

remove bmk_ prefix from routines private to platform/hw

hw: Initialise console first thing at boot

Otherwise we can't use bmk_printf() or bmk_platform_halt() in early init
code.

hw: Fix multiboot flag parsing

MULTIBOOT_MEMORY_INFO should be MULTIBOOT_INFO_MEMORY, these are two
different things.

Multiboot command line should only be used if MULTIBOOT_INFO_CMDLINE is
present.

+ bmk_{round,trunc}_page()

initialize bmk_core with page_shift instead of page_size

make bmk_memsize a variable instead of a function call

simplify memory location calculation

osbegin should be trunc_page()'d to get the size right

not round_page()-d!

don't assume that pointers are uint32_t-sized

don't assume that osbegin is page-aligned

Use the platform/xen page allocator. We can free pages now. Whee.

TODO:
* push implementation into bmk-core, share with xen
* test more

Rename baremetal -> hw, as discussed on rumpkernel-users

Migration: use "hw" where you would have previously used "baremetal".