system: sysupgrade: rework firmware validation

system: sysupgrade: rework firmware validation

Fixes following deficiencies:

 * unhandled read() errors
 * everything bundled in one long function, which is hard to follow and
   reason about
 * JSON parser errors are being ignored, anything else then
   json_tokener_continue is fatal error
 * JSON parser errors are being output to stderr, thus invisible via SSH
 * validate_firmware_image_call can fail at a lot of places, but we just
   get one generic "Firmware image couldn't be validated" so it's hard
   to debug

Cc: Rafał Miłecki <rafal@milecki.pl>
Tested-by: Kuan-Yi Li <kyli@abysm.org>
Signed-off-by: Petr Štetiar <ynezz@true.cz>