vsftpd: Add ftp user creation

In order to remove '/home/ftp' and the ftp user from the 'skeleton'
package, we need to add the creation of the ftp users to the package so
it still work out of the box (with an anonymous user).

[Thomas:
 - remove the VSFTPD_FIX_HOME_PERMISSIONS variable, and simply put the
   additional command in VSFTPD_INSTALL_TARGET_CMDS.
 - slightly tweak the comment.]

Signed-off-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Reviewed-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>