Relicense custom_* source code examples.

Explicitly permit all modifications to allow the intended use of these
files as an example and as a starting point for customization.
Add configure option to use external libcintelhex.

Introduce --with-cintelhex option to let the user provide an
installation prefix to check.  Also accept "internal" to use the
embedded copy, or "no" to disable.  The default is to auto-detect a
system-wide installation, then fall back to the embedded copy or
ultimately exclude Intel Hex support if all tests failed.
Conditionalize internal libcintelhex build.

Only include libcintelhex subdirectory in build and add the proper
tool flags when actually linking to it.
Honor HAVE_INTELHEX macro.

Conditionally compile code using the image_ihex module only when
HAVE_INTELHEX is enabled.
Conditionalize image_ihex module.

Avoid building the image_ihex module and linking to libcintelhex
unless HAVE_INTELHEX conditional is enabled.
Fix result messages for --enable-custom-*.
Merge branch 'optional-ihex' into develop