onie-boot:  Initial implementation of onie-boot-update

x86_64 based platforms use two commands for configuration the GRUB
configuration files:

  - onie-boot-update  -- generates the grub configuration file from fragments
  - onie-boot-default -- set the default GRUB menu entry and call onie-boot-update

These scripts are intended to be simple and runnable from just about
any OS using busybox.

onie-boot-update behaves as follows:

1. mount the ONIE-BOOT partition read-only on a temp directory

2. mount the ONIE-CONFIG partition read-write on a temp directory

3. create the final grub.cfg in the ONIE-CONFIG partition by
   concatenating the following:

   ONIE-BOOT/grub/grub-common.cfg  (boiler plate stuff, default serial console)
   ONIE-CONFIG/grub.d/*            (NOS menu entries and customizations)
   ONIE-BOOT/grub/grub-onie.cfg    (ONIE menu entries and fallback)

   The result is stored in ONIE-CONFIG/grub.cfg

   A sample contents of ONIE-CONFIG/grub.d looks like:

   grub.d/10_default  <= NOS installer sets, via onie-boot-default
   grub.d/20_custom   <= possibly blank - set serial parms
   grub.d/30_NOS_XXX  <= NOS installer adds menu entries here

4. umount ONIE-BOOT and ONIE-CONFIG

onie-boot-default behaves as follows:

usage: onie-boot-default <menu entry>

The following actions occur:

1. mount the ONIE-CONFIG partition read-write on a temp directory
2. create ONIE-CONFIG/grub.d/10_default with the specified menu entry
3. umount ONIE-CONFIG