syslinux.bbclass: make vm and live can be built together

syslinux.bbclass: make vm and live can be built together

* The vm image(hdddirect, vmdk, qcow2, vdi) and live image (hddimg, iso)
  couldn't be built together because the following vars settings are
  conflicted:
  - SYSLINUX_ROOT (/dev/sda2 vs /dev/ram0)
  - LABELS (boot vs boot install)
  - INITRD (None vs live install)
  - SYSLINUX_CFG (see above)
  Introduce new vars (SYSLINUX_ROOT_VM/_LIVE, the samilar to others) to
  make them can work together, now we can build all of them together:

  IMAGE_FSTYPES += "live iso hddimg hdddirect vmdk qcow2 vdi"

* Use SYSLINUX_CFG rather than SYSLINUXCFG to keep align with others
  SYSLINUX vars.

* The SYSLINUX_TIMEOUT had been set, but it didn't work since
  AUTO_SYSLINUXMENU wasn't set, this would cause confusions, so also set
  AUTO_SYSLINUXMENU.

* Move SYSLINUX_PROMPT and SYSLINUX_TIMEOUT to syslinux.bbclass rather
  than in separate classes since they are the same.

* Set SYSLINUX_TIMEOUT to 50 to have a unique timeout for syslinux.

[YOCTO #9161]

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>