Add basic exFAT support.

Several partners have been requesting exFAT support.  Android doesn't
natively support exFAT, but we're at least willing to try mounting an
exFAT filesystem if we detect the Linux kernel supports it, and if
helper binaries are present.

This CL is simple scaffolding, and it provides no actual
implementation of exFAT.

Test: builds, boots
Bug: 67822822
Change-Id: Id4f8ec3967b32de6e1c0e3c4b47fe6e43a6291ab
5 files changed