mailbox_client: add rpc mechanism and storage manager support

Revise the mailbox protocol between the SMC and the SEC. The SMC
writes rpc requests to the mailbox FIFO and optionally attaches
the physical address of a page frame for returning bulk data.

RPC messages are encoded with serde+postcard. Message are defined for:
- GetBuiltins -> Vec(String) of builtin filenames
- FindFile(name) -> (fid, size_bytes) locate a file in SPI flash
- GetFilePage(fid, offset) -> <data in attached page>
- Test(count) + <page> -> <modified page>

The matcha app now has a heap and global allocator (the default heap
size looks to be 1KiB).

Bug: 294433731

Change-Id: I115560455b3022f70e5488a4fd79c969b5cb1792
4 files changed