Makefile: export BUILD_DIR

$(BUILD_DIR) is a nice place to put files generated during the build.
With the advent of user-supplied step-hooks, they may want to store
some information on the build.

Export BUILD_DIR to that effect; update manual accordingly.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>