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
esptool_py defines command `esptool_py_flash_project_args` that
generates arg file for esptool.py. Two of the arguments are the offset
and image, which are not being used when a template file is given.
This commit makes variables OFFSET and IMAGE available to the template
file, which will holds the value of the offset and image arguments to
`esptool_py_flash_project_args`.
