DC Changes for Boot Menu

If you don't use distro-const(1M), you can ignore this e-mail.
If you are using build 166 or are in sync with the slim_source gate, my push for:

 o 7038421 Provide a boot checkpoint implementation based on pybootmgmt

is a flag day and manifests delivered must be used to build images.

This fix incorporates changes for all X86 DC installer manifests:

- the <grub_mods> element (and all sub-elements) have been removed and are no
  longer part of the XML schema.
 
- the <boot_mods> element (and its sub-elements) has been introduced as a
  more generic replacement for the <grub_mods> element. A new, corresponding
  boot_mods.dtd schema has also been introduced.

The standard boot menu customisations for the Solaris X86 Text, LiveCD and
Automated installers have been converted to use the new <boot_mods> elemenent
in their manifest files:
- /usr/share/distro_const/dc_text_x86.xml
- /usr/share/distro_const/dc_livecd.xml
- /usr/share/distro_const/dc_ai_x86.xml

Further customisation of the boot menu can be applied using the examples
documented in these files.

If there are any further questions, please let me know.

- Niall