file_util: consolidate hex/unhex routines

file_util: consolidate hex/unhex routines

Consolidate the various copies of the bin2hex table into one global variant
and replace the broken hex() define with a static helper function to prevent
accidential multiple expression evaluation.

This also fixes bug when converting hexadecimal checksums into binary forms
where the letters a..f/A..F where incorrectly converted into 0..5 instead
of 10..15 .

Signed-off-by: Jo-Philipp Wich <jo@mein.io>