am3517-evm: switch default u-boot preference to machine-specific tree

SDKs or other users of meta-ti can still use one of the other recipes for
stability or featureset, by setting PREFERRED_PROVIDER in their config.

Signed-off-by: Denys Dmytriyenko <denys@ti.com>

conf/machine/am3517-evm.conf

@@ -17,6 +17,10 @@ USE_VT = "2"
 
 PREFERRED_PROVIDER_virtual/kernel = "linux-omap-psp"
 
+# Support for am3517 is in the same tree as am33x
+PREFERRED_PROVIDER_virtual/bootloader = "u-boot-am33x"
+PREFERRED_PROVIDER_u-boot = "u-boot-am33x"
+
 UBOOT_MACHINE = "am3517_evm_config"
 XLOAD_MACHINE = "am3517evm_config"