Remove dialog from rescue Makefile.

dialog introduces unwanted dependencies in rescue.
If we want to add a binary to the installation ISO,
we should explicitly add it to the scripts which create
the install ISO, and not modify src/rescue/rescue/Makefile.

In FreeNAS, we cleaned up the create_iso.sh script in these changesets:
d29cbfcc8e9bed8510b59dd86e3e93f3c0e00ea2
db5e03a1451c6e3cac5bb3765897feda6ab940be

so that the install ISO takes dialog from /usr/bin/dialog and *not* /rescue/dialog.

(cherry picked from commit 7655ded674d77c1b4644e8e437167554ec4da430)
Revert "Build dialog(1) as part of rescue binary."

(cherry picked from commit 68310889d3c9a9a860b6817228fa8aae61969c1f)
Add a tunable for the default timeout.

(cherry picked from commit 65c945d5837493805e85471f2482c74e902d3cc2)
(cherry picked from commit b59d3d16c5e1ae498338512f1fa1e21ee7b6389d)
Revert "Add extra tools to the rescue image"

This reverts commit c7deb3988cbd01c34c79aaa7c4182aabbb5c92df.

This is not needed, because the extra tools were added
to the FreeNAS build/create_iso.sh script in b62da2ce813980107859e319cd4a174201aed683 .
In future, if we need to add new binaries to the install ISO, we should
add them in build/create_iso.sh instead of adding to the rescue image.

This minimizes differences to FreeBSD.

(cherry picked from commit 0a2c9faebb0e1bd2c22fcc780d0a709620950081)