binman: Allow disabling expanding an entry

binman: Allow disabling expanding an entry

At present there is a command-line flag to disable substitution of expanded
entries. Add an option to the entry node as well, so it can be controlled
at the node level.

Add a test to cover this. Fix up the comment to the checkSymbols() function
it uses, while we are here.

Signed-off-by: Simon Glass <sjg@chromium.org>