Partially revert "pxe: Pass absolute path to pxe_chdir()"
This partially reverts commit
b208ba467f678ed8e73f8d11fc0609634120cb83.
It isn't correct to always pass URL_OLD_TFTP to chdir(), since the
path prefix option may contain a full url, e.g. http://
Specialise the one case where we need to build a ::-style TFTP path
inside of get_prefix() to maintain backwards compatability, since from
Syslinux 5.00 onwards we may move out of the root directory ("::")
while searching for ldlinux.c32.
Signed-off-by: Matt Fleming <matt.fleming@intel.com>