cmd_elf.c: Cleanup bootvx and handle new CONFIG_SYS_VXWORKS parameters

- fix size too small by one in sprintf
- changed old (pre 2004) device name ibmEmac to emac
- boot device may be overriden in board config
- servername may be defined in board config
- additional parameters may be defined in board config
- fixed some line wrappings
- replaced  redundant MAX define by max

Signed-off-by: Niklaus Giger <niklaus.giger@member.fsf.org>