src/update_handler: generate only valid vfat label names
src/update_handler: generate only valid vfat label names

Depending on the vfat implementation, vfat labels can be interpreted
quite strictly, following the original specification.

This is why mkfs.vfat introduced more stricter sanity checks on label
names with dosfstoolfs v4.2.

See also dosfstools commit 40da1b2c ("fatlabel: Implement proper checks
when setting new label") for this:
https://github.com/dosfstools/dosfstools/commit/40da1b2c0129124f70fc0ec74363c699a3a479f8

Labels created so far by RAUC did not follow the FAT specification, too.
This becomes actually more critical as recent mkfs.vfat now refuses vfat
creation on the target.

This commit introduce vfat_label_generator() function that turns the
slot name into a valid FAT label.

This also makes the prior string length check superfluous which was
incorrect anyway as it tested for 16 instead of allowed 11 characters.

Signed-off-by: Enrico Joerns <a2e5f936c390f36494e46e71bcfbe365bb1e8c49@pengutronix.de>

rauc/rauc,jluebbe/rauc,rauc/rauc,jluebbe/rauc,rauc/rauc,ejoerns/rauc,jluebbe/rauc,ejoerns/rauc,ejoerns/rauc