aspeed: evb-ast2500: Use 2019.04-openbmc-aspeed u-boot branch

The old 2016.09 branch is unmaintained. ASPEED have support for the 2500
in the SDK, so use that instead.

Change-Id: Iece96616d24ac21028e91695e0c8ab86d9ade1a9
Signed-off-by: Joel Stanley <joel@jms.id.au>

meta-aspeed/conf/machine/evb-ast2500.conf

@@ -1,10 +1,7 @@
 KERNEL_DEVICETREE = "aspeed-ast2500-evb.dtb"
 
-PREFERRED_PROVIDER_virtual/bootloader = "u-boot-aspeed"
-PREFERRED_PROVIDER_u-boot = "u-boot-aspeed"
-PREFERRED_PROVIDER_u-boot-fw-utils = "u-boot-fw-utils-aspeed"
-
-UBOOT_MACHINE = "ast_g5_phy_config"
+UBOOT_MACHINE = "evb-ast2500_defconfig"
+UBOOT_DEVICETREE = "ast2500-evb"
 
 require conf/machine/include/ast2500.inc
 require conf/machine/include/obmc-evb-common.inc