Allow colon in PXE bootfile URLs

Allow colon in PXE bootfile URLs

- U-boot's PXE flow supports prefixing your bootfile name with an
  IP address to fetch from a server other than the DHCP server,
  e.g. `hostIPaddr:bootfilename`:
  https://github.com/u-boot/u-boot/commit/a93907c43f847f076dd0e34ee3b69b5e8e6d0d29
- However, this breaks bootfile paths which contain a colon, e.g.
  `f0:ad:4e:10:1b:87/7/pxelinux.cfg/default`
- This patch checks whether the `hostIPaddr` prefix is a valid
  IP address before overriding the serverIP otherwise the whole
  bootfile path is preserved

Signed-off-by: Lyle Franklin <lylejfranklin@gmail.com>