Make the slice names for root configurable. For embedded platforms, we
need s1 to be a FAT partition, s2 to be the config partition and s3
and s4 to be the ping-pong upgrade partitions.

NANO_SLICE_ROOT defaults to s1
NANO_SLICE_ALTROOT defaults to s2
NANO_SLICE_CFG defaults to s3
NANO_SLICE_DATA defaults to s4

All can be overridden in the config file. Some basic sanity checking
is in place, but is no substitute for being careful.