esx, aio, page-cache

Kernel ELF file: move from root filesystem to boot filesystem

This commit adds a new partition, containing the "boot filesystem",
and moves the kernel file from the existing root filesystem to the
new boot filesystem. This prevents the user process from accessing
(and possibly overwriting) the kernel file.
As a side effect, stage2, which now loads the boot filesystem, no
longer needs to load metadata for the entire root filesystem, which
was unnecessary overhead at boot time.
The location of the root filesystem is no longer derived from the
REGION_FILESYSTEM memory region, but is found from the MBR
partition table.
The contents of the boot filesystem are specified in the manifest
file via the "boot" tuple. If such tuple is not found in a given
manifest, for backward compatibility the mkfs tool retrieves the
kernel from the root tuple.