read_spec_entry: fail on non-ascii

Inserting non-ascii characters into the following files:
 * file_contexts
 * property_contexts
 * service_contexts
can cause a failure on labeling but still result in a successful
build.

Hard error on non-ascii characters with:
<path>:  line 229 error due to: Non-ASCII characters found

Change-Id: Iac1c95a90e840e47f46cadca33fcc79cfdbe8618
Signed-off-by: William Roberts <william.c.roberts@intel.com>