Fixed some problems with libnvram

- On board-qemu, the logging partitions were too big (bigger than the total
  NVRAM size).
- Fixed a compiler warning about type-punned pointers in nvram.c
- When DISABLE_NVRAM is set, the fake buffer should not be accessed with
  cache-inhibited functions
- Makefile did not generate proper dependencies

Signed-off-by: Thomas Huth <thuth@linux.vnet.ibm.com>