hikey: Build boot_fat.uefi.img only if TARGET_BOOTIMAGE_USE_FAT=true am: e8886ddf2e am: 9888d33ae1
am: bc5e870e20

* commit 'bc5e870e204d225b32b8f541df5663b32d32951b':
  hikey: Build boot_fat.uefi.img only if TARGET_BOOTIMAGE_USE_FAT=true

Change-Id: I4fc28c9e58e8ea60567e008e169a3adbae781cf4