plat(fvp): use generic image fetching functions

Some generic functions have been provided for fetching prebuilt images -
use these for FVP runs.

Signed-off-by: Harrison Mutai <harrison.mutai@arm.com>
Change-Id: I785890c9b0370279230dfd6f71a54bcdc0ebc9c3
diff --git a/run_config/fvp-fip.uboot b/run_config/fvp-fip.uboot
index 62957a9..562fa2a 100644
--- a/run_config/fvp-fip.uboot
+++ b/run_config/fvp-fip.uboot
@@ -6,6 +6,6 @@
 #
 
 post_tf_build() {
-	get_uboot_bin
+	url="$uboot_url" filename="uboot.bin" fetch_and_archive
 	build_fip BL33="$archive/uboot.bin"
 }