In the manual it's said that ISP is possible wiht USB, is there a linux commandline tool or so where I can give my .axf file as an argument and program the flash? It's a LPC546xx
Is mass storage boot a viable way for you to work? Then the device will just appear like a mass storage drive; you delete the firmware.bin file you will see there, then replace it with your application, renamed to firmware.bin.
Not sure why you have to have .axf files ... MCUXpresso IDE will generate a bin file for you automatically - right click on the axf file and select Binary Utilities > Create binary to get it.