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.
genode_until_run can be called now with a spawn id to able to reattach to a
spawned process (amt, serial output). Run scripts can now call genode_until_run
multiple times.
