Implement new MFS_IMAGE_UZIP kernel makeoption, fix MFS_IMAGE without MD_ROOT_SIZE on MIPS*

[sobomax]

Summary

o Implement new MFS_IMAGE_UZIP option to request automatic
  compression of the image using mkuzip(8) utility. This saves
  time when doing iterative development. As a bonus, MFS_IMAGE
  is set as a dependency for the kernel, so if that one gets
  modified, it should also trigger re-compilation even in the
  NO_CLEAN mode.

o Document MFS_IMAGE_UZIP, add some more documentation for the
  case when MD_ROOT_SIZE is omitted;

o Fix compilation of MIPS MFS_IMAGE kernels without explicit
  MD_ROOT_SIZE specifier.

Test Plan

Tested by buildworld+buildkernel on mips:mips, mips:mipsel, mips:mips64 and mips:mips64el

[wblock]

share/man/man4/md.4

75

"would" is kind of unsure. Maybe just

This statically reserves an 8MB area in the kernel and puts...

79

How about

The
.Dv MD_ROOT_SIZE
parameter can be omitted, and the size of the RAM disk will be
determined automatically from the actual size of the

86

s/ramdisk/RAM disk/

90

This sentence is really long and has a lot of conditions. Try to simplify:

When memory is limited and read-write access to the RAM disk data is not needed,
.Xr mkuzip 8
can be used to compress the image before it is linked to the kernel:

101

Not yours, but "to use" is not necessary.