Update mailbox to support 'short' (28-byte) and 'long' (4k) messages.

MatchaApp has a trivial message-handling loop that just edits messages and sends them back.
Added support for async wait in MatchaApp, implemented in mailbox_client.rs
I also ran "cargo fmt", which shuffled a few "use" lines.
MailboxHAL is now static_init'd by MatchaPlatform and components/capsules can hold a static ref to it.
Mailbox IRQs now go through a trivial ISR trait.
Platform-specific mailbox constants moved to matcha/config/src/lib.rs

Change-Id: I8860a76b4b8e3c712206dff61bfb1faee156a657
9 files changed