[riscv] Add a .pf32 build target for padded parallel flash images

QEMU's -pflash option requires an image that has been padded to the
exact expected size (32MB for all of the supported RISC-V virtual
machines).

Add a .pf32 build target which is simply the equivalent .sbi target
padded to 32MB in size, to simplify testing.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/Makefile

@@ -26,6 +26,7 @@ PRINTF		:= printf
 PERL		:= perl
 PYTHON		:= python
 TRUE		:= true
+TRUNCATE	:= truncate
 CC		= $(CROSS_COMPILE)gcc
 CPP		= $(CC) -E
 AS		= $(CROSS_COMPILE)as