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
!4452 and !4897 made some ill-advised corrections to dependency info;
revert those in this MR. Handling pre-built binaries as imported
libraries is retained, however.
