MFC r309412,r316109,r316132:

r309412 (by imp):

dd is currently a bootstrap tool. It really doesn't have any business
being a bootstrap tool. However, for reproducible build output,
FreeBSD added dd status=none because it was otherwise difficult to
suppress the status information, but retain any errors that might
happen. There's no real reason that dd has to be a build tool, other
than we use status=none unconditional. Remove dd from a bootstrap tool
entirely by only using status=none when available. This may also help
efforts to build the system on non-FreeBSD hosts as well.

r316109:

Don't hardcode input files for stage 1/2 bootloaders; use .ALLSRC instead

This is a better pattern to follow when creating the bootloaders and doing
the relevant space checks to make sure that the sizes aren't exceeded (and
thus, copy-pasting is a bit less error prone).

r316132:

Parameterize out 7680 (15 * 512) as BOOT2SIZE, similar to sys/boot/i386/zfsboot/...

This is being done to make it easier to change in the future--this action might be
needed sooner rather than later because of gcc 6.3.0 bailing, stating that there
is negative free space left (deficit) in the boot2 bootloader.

MFC r264400,r265836:

r264400:

NO_MAN= has been deprecated in favor of MAN= for some time, go ahead
and finish the job. ncurses is now the only Makefile in the tree that
uses it since it wasn't a simple mechanical change, and will be
addressed in a future commit.

r265836:

Remove last two NO_MAN= in the tree. In both of these cases, MAN= is
what is needed.

MFC r264400,r265836:

r264400:

NO_MAN= has been deprecated in favor of MAN= for some time, go ahead
and finish the job. ncurses is now the only Makefile in the tree that
uses it since it wasn't a simple mechanical change, and will be
addressed in a future commit.

r265836:

Remove last two NO_MAN= in the tree. In both of these cases, MAN= is
what is needed.