obmc-phosphor-image: Improper use of DATETIME

The use of the DATETIME variable has issues with the sstate cache
because of the variable changing values, and therefore the checksum
used for sstate calculation also changing.

Other existing recipes add DATETIME to the 'vardepsexclude' list.
This case is glossed over very rapidly in section 21.3.2 "Checksums"
of the Mega Manual.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meta-phosphor/classes/obmc-phosphor-image_types_uboot.bbclass b/meta-phosphor/classes/obmc-phosphor-image_types_uboot.bbclass
index adb86e0..c44139e 100644
--- a/meta-phosphor/classes/obmc-phosphor-image_types_uboot.bbclass
+++ b/meta-phosphor/classes/obmc-phosphor-image_types_uboot.bbclass
@@ -14,6 +14,7 @@
 INITRD_LINK_NAME = "${INITRD_IMAGE}-${MACHINE}${INITRAMFS_FSTYPE}"
 
 FLASH_IMAGE_NAME ?= "flash-${MACHINE}-${DATETIME}"
+FLASH_IMAGE_NAME[vardepsexclude] = "DATETIME"
 FLASH_IMAGE_LINK ?= "flash-${MACHINE}"
 
 FLASH_UBOOT_OFFSET ?= "0"
@@ -95,3 +96,4 @@
        tar -h -cvf ${ddir}/${MACHINE}-${DATETIME}.all.tar -C ${ddir} image-bmc
        tar -h -cvf ${ddir}/${MACHINE}-${DATETIME}.tar -C ${ddir} image-u-boot image-kernel image-initramfs image-rofs image-rwfs
 }
+do_generate_flash[vardepsexclude] = "DATETIME"