python2: Fix disabling module bsddb

Using this minimal defconfig

BR2_TOOLCHAIN_EXTERNAL=y
BR2_PACKAGE_PYTHON=y
BR2_PACKAGE_BERKELEYDB=y

the current code does not prevent the build of

output/build/python-2.7.6/Modules/_bsddb.o

because the module is really called _bsddb,
see python-2.7.6/Modules/_bsddb.c, line 9604.

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>