beagle_x15: use prebuilt u-boot

In order to remove final arm32 gcc cross compiler, we need to be able to
build u-boot with Clang, but are facing:
https://bugs.llvm.org/show_bug.cgi?id=44562
Until that is fixed, move to a prebuilt binary for u-boot to unblock
removing old GCC as per:
https://android.googlesource.com/platform/prebuilts/clang/host/linux-x86/+/master/GCC_4_9_DEPRECATION.md

Bug: 147296060
Test: lunch beagle_x15 && m && find $OUT -name MLO && \
  find $OUT -name u-boot.img
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Change-Id: I848dba99e622e07560888a62d84bc9cb55f92050
3 files changed