Add HLK-RM04 and some options to build it correctly. Also enable libs needed for telnetd

[anantkamath]

• Added a cfg for the HLK-RM04 which uses the RT5350 kernconf.
  (ref: http://permalink.gmane.org/gmane.os.freebsd.devel.mips/4156)
  A separate rt5350 config can perhaps be added later
• The build_ralink script adds a magic string/preamble string to the uboot image that is not necessary for all devices. So added a config option for it : X_UBOOT_AIRSTATION_PREAMBLE
  Only fon2305e used build_ralink, so enabled this option for it
• Also added an option for configuring the console tty: X_CFG_DEFAULT_TTY
  (which is ttyu1 and not ttyu0 for rt5350 based devices)
• Enabled the commented out libs needed for telnetd to work in build_mfsroot and added another one that is necessary

[erikarn]

why do you need the kerberos libraries? You shouldn't for AP builds.

[anantkamath]

You're right, I was building with mipsel instead of mipsel_ap. The libs for telnetd aren't required with mipsel_ap.

Have updated the PR with these new changes.
I can reduce the number of commits by a couple or more by rebasing my branch/ squashing some of them if you feel there are too many and everything else looks okay.