support variable-size SPI chip for dummy programmer

This is designed for firmware updater to pack firmware image
preserving some specific partitions in any size.

BUG=none
TEST=ran the command line below:
$ flashrom -p dummy:image=${TMP_FILE},size=16777216, \
emulate=VARIABLE_SIZE -w ${IMG} -V -f

$ flashrom -p dummy:image=${TMP_FILE},size=auto, \
emulate=VARIABLE_SIZE -w ${IMG} -V -f

Signed-off-by: Namyoon Woo <namyoon@google.com>
Change-Id: Iff266e151459561b126ecfd1c47420b385be1db2
Reviewed-on: https://review.coreboot.org/c/flashrom/+/44879
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Edward O'Callaghan <quasisec@chromium.org>

flashchips.c

@@ -18757,6 +18757,27 @@ const struct flashchip flashchips[] = {
 		.read		= NULL,
 	},
 
+	{
+		.vendor		= "Generic",
+		.name		= "Variable Size SPI chip",
+		.bustype	= BUS_SPI,
+		.manufacture_id	= PROGMANUF_ID,
+		.model_id	= PROGDEV_ID,
+		.total_size	= 64,  /* This size is set temporarily */
+		.page_size	= 256,
+		.tested		= TEST_OK_PREW,
+		.probe		= probe_variable_size,
+		.block_erasers	=
+		{
+			{
+				.eraseblocks = { {64 * 1024, 1} },
+				.block_erase = spi_block_erase_d8,
+			}
+		},
+		.write		= spi_chip_write_256,
+		.read		= spi_chip_read,
+	},
+
 	{
 		.vendor		= "Generic",
 		.name		= "unknown SPI chip (RDID)",