Warn when ubootenv.d snippet will be ignored

(Closes: #884376)

Previously if you dropped in an bootscript snippet in
/etc/flash-kernel/ubootenv.d/ and the bootscr.* template
did not contain @@UBOOT_ENV_EXTRA@@ (which most don't)
the snippet would be silently ignored.

Warn the user and give some hints on where to look for the
solution in the warning message.