ks: stop using depricated "extlinux" keyword

ks: stop using depricated "extlinux" keyword

We renamed the "extlinux" installer framework keyword to "bootloader" in MIC.
MIC still supports both, but the intention is to remove the old one.

Lets start using "bootloader" keyword from now on in IVI.

This should fix the following MIC warning:

"please, use 'bootloader' installer framework plugin name instead of
'extlinux', the latter is obsolete and will be removed in future releases"

Change-Id: I78a425c08fef336ebac54070bf70bf773e466033
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>