PPC: Add u-boot firmware for e500

This adds a special build of u-boot tailored for the e500 platforms we
emulate. It is based on the current version of upstream u-boot which
contains all the code necessary to drive our QEMU provided machines.

Signed-off-by: Alexander Graf <agraf@suse.de>
diff --git a/configure b/configure
index dfbe75e..9d50189 100755
--- a/configure
+++ b/configure
@@ -5213,6 +5213,7 @@
     $source_path/pc-bios/*.dtb \
     $source_path/pc-bios/*.img \
     $source_path/pc-bios/openbios-* \
+    $source_path/pc-bios/u-boot.* \
     $source_path/pc-bios/palcode-*
 do
     FILES="$FILES pc-bios/`basename $bios_file`"