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]

(From OE-Core rev: e38c94d6bf83ed3ca7f046d9503e81b927487bf2)

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/boot-directdisk.bbclass

@@ -53,14 +53,13 @@ def pcbios_class(d):
 PCBIOS = "${@pcbios(d)}"
 PCBIOS_CLASS = "${@pcbios_class(d)}"
 
+# Get the build_syslinux_cfg() function from the syslinux class
 inherit ${PCBIOS_CLASS}
 inherit ${EFI_CLASS}
 
-# Get the build_syslinux_cfg() function from the syslinux class
-
 DISK_SIGNATURE ?= "${DISK_SIGNATURE_GENERATED}"
-SYSLINUX_ROOT ?= "root=/dev/sda2"
-SYSLINUX_TIMEOUT ?= "10"
+SYSLINUX_ROOT_VM ?= "root=/dev/sda2"
+SYSLINUX_CFG_VM  ?= "${S}/syslinux_hdd.cfg"
 
 boot_direct_populate() {
 	dest=$1
@@ -162,6 +161,7 @@ build_boot_dd() {
 python do_bootdirectdisk() {
     validate_disk_signature(d)
     if d.getVar("PCBIOS", True) == "1":
+        syslinux_set_vars(d, 'VM')
         bb.build.exec_func('build_syslinux_cfg', d)
     if d.getVar("EFI", True) == "1":
         bb.build.exec_func('build_efi_cfg', d)