net-snmp: install net-snmp-config script to STAGING_BINDIR_CROSS

This recipe was installing net-snmp-config script to STAGING_BINDIR
instead of STAGING_BINDIR_CROSS, during the do_install. It is not
recommended and must be done after staging.

Changes:
* Remove STAGING_BINDIR manipulations from do_install_append
* Add net_snmp_sysroot_preprocess function instead
* Append net_snmp_sysroot_preprocess to SYSROOT_PREPROCESS_FUNCS

Signed-off-by: Alexandre Fournier <alexandre.fournier@kiplink.fr>
Signed-off-by: Joe MacDonald <joe@deserted.net>