Support "updatable groups".

* BOARD_SUPER_PARTITION_GROUPS defines a list of "updatable groups". Each
updatable group is a group of partitions that share the same pool of free
spaces.

* For each group in BOARD_SUPER_PARTITION_GROUPS, a BOARD_{GROUP}_SIZE and
BOARD_{GROUP}_PARTITION_PARTITION_LIST may be defined.
    - BOARD_{GROUP}_SIZE: The maximum sum of sizes of all
      partitions in the group.
      If empty, no limit is enforced on the sum of sizes for this group.
    - BOARD_{GROUP}_PARTITION_PARTITION_LIST: the list of partitions that
      belongs to this group.
      If empty, no partitions belong to this group, and the sum of sizes is
      effectively 0.

* BOARD_SUPER_PARTITION_PARTITION_LIST should not be defined
by the device. It is now computed from all
BOARD_{GROUP}_PARTITION_PARTITION_LIST.

* Each 'updatable group' has its own pool of space for its
partitions to grow into. Enforce the following:
    * sum(all partitions) <= super partition (/ 2 for A/B)
    * For each group, sum(partitions in group) <= group size
    * sum(all group sizes) <= super partition (/ 2 for A/B)

Test: builds
Bug: 111610495
Change-Id: I072b011714ec31a1d8813cc75edd27da3c6ff39a
Merged-In: I072b011714ec31a1d8813cc75edd27da3c6ff39a