src.sys.mk: Support src.conf in SRCTOP

If SRCCONF is not defined and src.conf exists at the top level of the
source tree, use that instead of /etc/src.conf.

MFC after: 3 days
Reviewed by: kevans, imp
Differential Revision: https://reviews.freebsd.org/D52470

(cherry picked from commit dd8c666d8b78f6b9ddb691f0505837fa885ff3b4)

UPDATING: mention the src.conf change

Fixes: dd8c666d8b78 ("src.sys.mk: Support src.conf in SRCTOP")
(cherry picked from commit ab2fea3f9a5a58f0ba90499ad3a8614e3dca0c5f)