net-snmp: fix some invalid paths

Because the package is not built in place @srcdir@ is
an absolute path to the source directory instead of ".".

Because of this some target scripts like  net-snmp-create-v3-user
and net-snmp-config that are using thisvariable in their *.in
files (NSC_SRCDIR=@srcdir@) contain invalid 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>