binman: Don't generate an error in 'text' entry constructor

It is not good practice to raise an exception in a constructor. In this
case the 'text' entry may not actually be used, if -i is used to filter
out the images that get built.

Move the exception to where the data is actually used.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
1 file changed