run: support TFTP servers using absolute paths

TFTP server requiring absolute directory names are supported (better) -
specify in RUN_OPT "--tftp-absolute" to create Pulsar config with absolute
path names for PXE boot.

Additional a symbolic link is created from the build directory to
"$PXE_TFTP_DIR_BASE$PXE_TFTP_DIR_OFFSET" automatically. This eases the use
together with autopilot for x86.