depends: create a hostid and buildid and add option for salts

These add very simple sanity checks to ensure that the build/host toolchains
have not changed since the last run. If they have, all ids will change and
packages will be rebuilt.

For more complicated usage (like parsing dpkg), HOST_ID_SALT/BUILD_ID_SALT may
be used to introduce arbitrary data to the ids.

depends/README.md

@@ -38,6 +38,8 @@ The following can be set when running make: make FOO=bar
     NO_WALLET: Don't download/build/cache libs needed to enable the wallet
     NO_UPNP: Don't download/build/cache packages needed for enabling upnp
     DEBUG: disable some optimizations and enable more runtime checking
+    HOST_ID_SALT: Optional salt to use when generating host package ids
+    BUILD_ID_SALT: Optional salt to use when generating build package ids
 
 If some packages are not built, for example `make NO_WALLET=1`, the appropriate
 options will be passed to bitcoin's configure. In this case, `--disable-wallet`.