Makefile: revert BUILD_DIR export

Revert commit feb08770
U-Boot uses $(BUILD_DIR) and causes issues, other packages might as
well. Fixes:
http://autobuild.buildroot.net/results/1f6/1f65289d5c284caa82a4e9137613f792797ee08a/
http://autobuild.buildroot.net/results/6a5/6a5f77483e908dc96d08824cf9113402431bace7/



It should probably be named somewhat more uniquely.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>