post-{build, images} hooks: export BUILD_DIR too

Also export BUILD_DIR for post-{build,images} hooks, so they do have
a place to store generated files.

Note: this will be more einteresting for the instrumentation of steps,
to come in a later patch.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas De Schampheleire <patrickdepinguin@gmail.com>
Reviewed-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Reviewed-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>