Add definition for hb_lid_ids attribute to string_attrs.json

The eBMC for IBM systems will have copies of all the files needed
to support any type of sytem. The host however, only wants 'this'
systems copy of a given file, such as the device layout information.
This attribute gives a way to the eBMC to communicate which .lid files
the host should access without the host boot firmware having to keep
a list of all the possible lid files for the various system types and
knowing what type of system it is operating in.

The format of this attribute will be a comma separated list of pairs:
  <readable name A>=<lid id 1>,<readable name B>=<lid id 2>

Example :
  HBI=81e0065d,HBB=81e0065a

Signed-off-by: Christian Geddes <crgeddes@us.ibm.com>
Change-Id: I9d5c600d7fd8ea6d2efc37ce41efc57d4e1c8f62