buildroot

berkeleydb : convert to autotools infrastructure & bump to 4.4.20

Changes by Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

 * Installation to target is the default
 * Use BERKELEYDB_SUBDIR to get rid of the specific BUILD_CMDS and
   INSTALL_STAGING_CMDS
 * Split the INSTALL_TARGET_CMDS to install development files on the
   target only when needed

Signed-off-by: cmchao <cmchao@gmail.com>