ANDROID: setlocalversion: make KMI_GENERATION optional

GKI required the KMI_GENERATION to be added to the kernel version
string, but this only makes sense for GKI kernels, for non-GKI kernels
we don't need it. Leave all the other stuff we added, though, as it
seems useful.

Bug: 205897686
Change-Id: I2e7b3cb7ed5529f1e5e7c9d79a1f7ce4a1b6ee1f
Signed-off-by: Alistair Delva <adelva@google.com>