loader: Drop the HashMap for partition lookup by id

The HashMap constructor was called before the heap is initialized,
ending up calling malloc from the OpenHashMap constructor.

Oddly it was still working on x86 but broke other platforms.

Instead we add a Lookup() static method to Partition,
which by default walks gPartitions for the id,
and recursively calls itself on the children lists.

This means we must add a partition even temporarily to gPartitions
before Scan()ing it though.

Signed-off-by: François Revol <revol@free.fr>