Given the binary target generated by the idf_build_binary or
idf_sign_binary function, add a new POST_BUILD dependency that displays
a message indicating the binary image must be manually signed before
flashing. This situation arises when CONFIG_SECURE_SIGNED_APPS is
enabled, but CONFIG_SECURE_BOOT_BUILD_SIGNED_BINARIES is not set. As a
result, the binary image is not signed during the build process and
requires manual signing.
Signed-off-by: Frantisek Hrbata <frantisek.hrbata@espressif.com>