Add a means to specify that the VM_POOL_SIZE can be 1gb and not 2gb

Due to a reported problem where the reserving of 2gb was too big under
certain circumstances then in order to provide a means for people who
hit this problem a define is added so that the VM_POOL_SIZE is set to be
1gb instead. This is an optional define and the default is for the size
to be 2gb which keeps it to current behavior.

Change-Id: I73200e1bcfcf7b37c6ba62f8b7bcf217f239dfcb
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@digia.com>