Makefile: Add a target for the Orange Crab v0.21 with LFE5U-85F

Makefile: Add a target for the Orange Crab v0.21 with LFE5U-85F

The existing orange crab target is for an older board with a
LFE5UM5G-85F device.  Newer orange crab boards (v0.21) have a
LFE5U-85F device in the -8 speed grade, so make a new target for them
called ORANGE-CRAB-0.21.

Also add flags to ecppack to indicate that the bitstream should be
compressed and can be loaded at 38.8MHz.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>