hikey: Build boot_fat.uefi.img only if TARGET_BOOTIMAGE_USE_FAT=true
am: e8886ddf2e

* commit 'e8886ddf2eeea5b49b11810696407dabaf66b7f7':
  hikey: Build boot_fat.uefi.img only if TARGET_BOOTIMAGE_USE_FAT=true

Change-Id: I6ec6315b3e7a50e7b949a3dbf2fecd1f28cd8c1f