core-image-lsb: enforce pam as a needed distro feature

core-image-lsb: enforce pam as a needed distro feature

core-image-lsb only gave a warning:
"WARNING: Building libpam but 'pam' isn't in DISTRO_FEATURES,
PAM won't work correctly"
when the proper DISTRO was not set for it.
default choice would be DISTRO = "poky-lsb",
but not necessarily, depending on each custom distro.

This fix will enforce the proper usage of pam
as a distro feature for core-image-lsb by giving
an error instead of just a warning.

Fixes [YOCTO #6073]

Signed-off-by: Cristian Iorga <cristian.iorga@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>