The second section of the buildfile starts with the [+script] attribute — this tells mkifs that the specified file is a script file, a sequence of commands that you want procnto to execute when it's completed its own startup.
In order to run a command, its executable must be available when the script is executed. You can add the executable to the image or get it from a filesystem that's started before the executable is required. The latter approach results in a smaller image.
In this case, the script file is an inline file (again indicated by the open brace). The file (which happens to be called ".script") contains the following:
procmgr_symlink ../../proc/boot/libc.so.3 /usr/lib/ldqnx.so.2 devc-ser8250-abc123 -F -e -c14745600 -b115200 0xc8000000 ^2,15 & reopen display_msg Serial Driver Started
This script file begins by creating a symbolic link to ../../proc/boot/libc.so.3 called /usr/lib/ldqnx.so.2.
Next the script starts a serial driver (the fictional devc-ser8250-abc123) in edited mode with hardware flow control disabled at a baud rate of 115200 bps at a particular physical memory address. The script then does a reopen to redirect standard input, output, and error. The last line simply displays a message.
As mentioned above, the bootstrap file can set the _CS_PATH and _CS_LIBPATH configuration strings. You can set PATH, LD_LIBRARY_PATH, and other environment variables if the programs in your script need them.