libedit: Make docs generation deterministic

The presence or lack of nroff on the host was changing the doc type. Stop
the code from looking at host paths outside HOSTTOOLS and hence cause the
doc type to be deterministic and reproducible.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 918e2b26)
Signed-off-by: Steve Sakoman <steve@sakoman.com>