support/mkusers: allow comments in users tables

The format of the users table files is non trivial, so it is sometimes
handy to add comments explaining the syntax (or simply the reason for
the user) inline in the files.

Ignore empty lines and comment lines prefixed with '#' similar to shell
or makedevs files.

Packages that defined no user (the vast majority) would cause an empty
line to be present in the internal users table, hence the reason we
skipped empty usernames. Now that we ignore empty lines, we no longer
need to check for empty usernames.

Reported-by: Peter Korsgaard <jacmet@uclibc.org>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Peter Korsgaard <jacmet@uclibc.org>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>