freedom-u540.conf: Define virtual/bootloader to be u-boot
Fixes
NOTE: Multiple providers are available for virtual/bootloader (u-boot, u-boot-juno, u-boot-orangepi-i96, u-boot-poplar, u-boot-socfpga)
Consider defining a PREFERRED_PROVIDER entry to match virtual/bootloader
Signed-off-by: Khem Raj <raj.khem@gmail.com>