Increase the size of riscv GENERICSD images to 6 GB

The stable/13 snapshot this week failed to build the riscv GENERICSD
image because it ran out of space.  Checking main and stable/14
snapshots, they are also low on space, around 100% or more of
capacity.  Increase them all from 5 GB to 6 GB.  Note, this is the
only riscv image configuration.

Discussed with:	cperciva

release/riscv/GENERICSD.conf

@@ -7,7 +7,7 @@ EMBEDDED_TARGET="riscv"
 EMBEDDEDBUILD=1
 FAT_SIZE="54m -b 8m"
 FAT_TYPE="16"
-IMAGE_SIZE="5120M"
+IMAGE_SIZE="6144M"
 KERNEL="GENERIC"
 MD_ARGS="-x 63 -y 255"
 PART_SCHEME="GPT"