discover/syslinux-parser: consistent cmdline arg processing

In signed-boot environments consistent handling of kernel commandline
options is essential as they must be pre-signed.  In the syslinux parser
ensure that in the absence of a global APPEND they are processed
exactly as found and not with the leading space that the current APPEND
processing has as a shortcut.

Signed-off-by: Brett Grandbois <brett.grandbois@opengear.com>
Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>