net-snmp: fix invalid paths in target net-snmp-config

Only net-snmp-config used for sysroot should have sysroot
specific paths.

Signed-off-by: Catalin Enache <catalin.enache@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Joe MacDonald <joe_macdonald@mentor.com>