evb-ast2500: set FLASH_SIZE for obmc-phosphor-image_types_uboot

obmc-phosphor-image_types_uboot constructs a full flash image from the
built kernel, initramfs, etc.  The size of the flash image depends on
FLASH_SIZE which is expected to be set by the machine config.
evb-ast2500 never set this which caused the initial setting of the flash
image to all 0xFF to silently fail.  A functional flash image was still
produced since the copying of the various files into the flash image
succeeded.  The flash image just ends early and writes zeros to unused
sections instead of 0xFF.

Change-Id: I640b2191e116ea82546a636e83c2a45c7e45c789
Signed-off-by: Rick Altherr <raltherr@google.com>
diff --git a/meta-openbmc-machines/meta-evb/meta-evb-aspeed/meta-evb-ast2500/conf/machine/evb-ast2500.conf b/meta-openbmc-machines/meta-evb/meta-evb-aspeed/meta-evb-ast2500/conf/machine/evb-ast2500.conf
index 3cadc65..3d88c87 100644
--- a/meta-openbmc-machines/meta-evb/meta-evb-aspeed/meta-evb-ast2500/conf/machine/evb-ast2500.conf
+++ b/meta-openbmc-machines/meta-evb/meta-evb-aspeed/meta-evb-ast2500/conf/machine/evb-ast2500.conf
@@ -3,5 +3,7 @@
 
 UBOOT_MACHINE = "ast_g5_phy_config"
 
+FLASH_SIZE = "32768"
+
 require conf/machine/include/ast2500.inc
 require conf/machine/include/obmc-bsp-common.inc