hikey: Drop support for 3.18 kernel

There are no more kernel binaries for this kernel so it does not make
sense to handle TARGET_KERNEL_USE=3.18. Remove the code, and custom
fstab.

Change-Id: I15dd43d4e5fb4acfcc15a256d1a5555f57cbc1f6
Signed-off-by: Alistair Strachan <astrachan@google.com>
3 files changed