syslinux.bbclass: Enable additional kernel parameters for syslinux

syslinux.bbclass: Enable additional kernel parameters for syslinux

Add additional parameter 'SYSLINUX_KERNEL_ARGS' in order to allow
for specific kernel parameters to be set when using syslinux.

The extra kernel parameters are added to btype[1] and then written out
as part of the APPEND field.

Signed-off-by: Konrad Scherer <Konrad.Scherer@windriver.com>
Signed-off-by: Mark Hatle <mark.hatle@windriver.com>
Signed-off-by: Hongxu Jia <hongxu.jia@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>