esmtp: update sendmail link files

esmtp use 'sbinsendmail' to define alternative ${sbindir}/sendmail, but
other packages msmtp and postfix use 'sendmail'. When remove esmtp, it
removes ${sbindir}/sendmail even msmtp or postfix is installed which has
alternative ${sbindir}/sendmail.

Make esmtp use 'sendmail' too to fix this issue.

Remove ${libdir}/sendmail which is only used by LSB core test for
historical reasons. And only create link file with fixed path
/usr/lib/sendmail for LSB images even for 64 bits system.

Signed-off-by: Kai Kang <kai.kang@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Joe MacDonald <joe_macdonald@mentor.com>