Implement FPM policy validation algorithm

@mcy is already working on this.

Since Sep 17, 2020.

Pesudocode for the algorithm:
```rust
fn verify_policy(
fpm: Fpm,
flash: &impl Flash,
hash_builder: &impl ha256::Builder
) -> Result {
for fw in fpm.fw_versions {
let version = flash.read(fw.version_addr, fw.version_len);
if version != fw.version {
continue
}
let mut sha = hash_builder.new();
for (ptr, len) in fw.signed_regions {
// Note: this is grossly exaggerated. There is an expectation that
// data will be fed into the hashing engine in small packets (O(1K),
// for example.
sha.write(flash.read(ptr, len));
}
if fw.signed_region_hash != sha.finish() {
return Err("hash mismatch");
}

let unused_regions = regions_except(flash, [fw.signed_regions,
fw.write_regions]);
for (ptr, len) in unused_regions {
for byte in flash.read(ptr, len) {
return Err("bad byte in blank region");
}
}
return fw
}
return Err("failed to find an acceptable version")
}
```