sed: use ALTERNATIVE_LINK

sed: use ALTERNATIVE_LINK

update-alternatives.bbclass assumes /usr/bin/sed if ALTERNATIVE_LINK is not
specified, however sed has changed its default installation directory to
/bin/sed. This causes below errors at the first boot of lsb image:

update-alternatives: Error: cannot register alternative sed to /usr/bin/sed
since it is already registered to /bin/sed

This fixes [BUGID #478]

Signed-off-by: Kevin Tian <kevin.tian@intel.com>