Add onie-boot-entry-* commands

Add commands used by a NOS installer for manipulating the ONIE GRUB
configuration.

The user settable ONIE GRUB configuration files live in
ONIE-CONFIG/grub.d

These CLI commands shield the installer from manipulating the GRUB
config files directly.

onie-boot-entry-show   -- list and dump the current entries
onie-boot-entry-add    -- Add/update a GRUB entry
onie-boot-entry-remove -- remove a GRUB entry