fvp: Update instructions for building the bootloader

A new bootloader-only configuration has recently been added to the
reference platform stack, update the instructions accordingly.
This brings the following improvements:
- Much smaller tree (therefore faster to sync).
- Much faster to build, as only the relevant projects (U-Boot and
  TF-A) are built.
- No need to download the 32-bit toolchain any more (all the
  components are 64-bit only).

Test: boot fvp-eng using the updated instructions
Change-Id: I21057d285054eb1caa2800d016127e5b8121e9a4
1 file changed