bootimg_pcbios: cleanup _do_configure_syslinux function

bootimg_pcbios: cleanup _do_configure_syslinux function

This commit:

1. Removes all unrequired function parameters.
   The part parameter was kept due to it's potential
   future usage in _do_configure_syslinux function.
   part.fstype specifically may be used with the
   rootfstype kernel paramater.

2. Sets a default timeout to 500 if bootloader --timeout
   not specified. To avoid 'None' being placed
   as the value in resulting configuartion file.
3. Sets a default kernel parameter string if
   bootloader --append not specified. This also
   helps avoid 'None' being places as the value
   in resulting configuration file.
4. Replace all instances of

   cr_workdir, "/hdd/boot"

   with variable

   hdddir

   as it's set at the top of the function. No,
   need to re-implement what the variable is
   already defined to store.

Signed-off-by: Vincent Davis Jr <vince@underview.tech>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>