Makefile: export a few variables to help support scripts

Support scripts (in support/) may need to parse the .config file, so give
them an easy access to it, by exporting BUILDROOT_CONFIG with the fully-
qualified path to .config.

Also, post-build scripts may need to reference a few locations, so export
those, too.

Note: we export both O and BASE_DIR. Although they are the same, BASE_DIR
is used internally, while O is used on the command line, which makes it a
bit ambiguous to know which to use. As users use O= on the command line,
they will probably tend to use that in their post-build scripts.

Update doc accordingly.

[Peter: fixed typo]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
cc: Peter Korsgaard <jacmet@uclibc.org>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>