update-alternative.bbclass: add batch alternatives support

update-alternative.bbclass: add batch alternatives support

With new batch ability, we can use below simple 2 lines for multiple alternatives
updates:

ALTERNATIVE_LINKS = "${bindir}/cmd1 {sbindir}/cmd2 ..."
ALTERNATIVE_PRIORITY = "100"

Then for each command "/path/cmd" listed in ALTERNATIVE_LINKS, below is done
automatically:

    ${D}/path/cmd is renamed to ${D}/path/cmd.{PN}
    a new alternative named 'cmd' is created which:
        links /path/cmd to /path/cmd.{PN} with priority specified in ALTERNATIVE_PRIORITY

This way the recipe with multiple alternatives could be simplified a lot. There
are still some cases where above assumptions may break, but I expect more recipes
should benefit from this simple enhancement

Fix [BUGID #257]

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