fix(config): warn about unknown keys in the configuration

Keys that aren't part of the configuration were silently ignored, so a
misspelled key like linux_initrd instead of linux-initrd, or
menu_timeout instead of menu-timeout, quietly did nothing. Warn about
each ignored key with its full path while loading the configuration.

Also warn when an action or generator configures more than one kind,
as only the first one found is used.

crates/boot/Cargo.toml

@@ -15,6 +15,7 @@ edera-sprout-bls.path = "../bls"
 edera-sprout-parsing.path = "../parsing"
 hex.workspace = true
 jaarg.workspace = true
+serde_ignored.workspace = true
 sha2.workspace = true
 toml.workspace = true
 log.workspace = true