Do not include bootloader in flash target when secure boot is enabled.
Emit signing warning on all cases where signed apps are enabled (secure
boot and signed images)
Follow convention of capital letters for SECURE_BOOT_SIGNING_KEY variable, since it is
relevant to other components, not just bootloader.
Pass signing key and verification key via config, not requiring
bootloader to know parent app dir.
Misc. variables name corrections
Issue is that when users creates a custom bootloader from
$IDF_PATH/components/bootloader. Parent project build uses the copy but
bootloader subproject build uses the original still. The issue is solved
by passing the custom bootloader as extra component directory so
bootloader build knows to use the new copy (itself) in the build.
Refactor IDF "project" functionality under a wrapping of the default
"project" command, so we can tweak it a bit...
Will need more testing in other environments.
