powerpc: Add "force config cmd line" Kconfig option

powerpc uses early_init_dt_scan_chosen() from common fdt code. By
enabling this option, the common code can take the built in
command line over the one that is comming from bootloader / DT.
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/Kconfig

@@ -618,6 +618,15 @@ config CMDLINE
 	  some command-line options at build time by entering them here.  In
 	  most cases you will need to specify the root device here.
 
+config CMDLINE_FORCE
+	bool "Always use the default kernel command string"
+	depends on CMDLINE_BOOL
+	help
+	  Always use the default kernel command string, even if the boot
+	  loader passes other arguments to the kernel.
+	  This is useful if you cannot or don't want to change the
+	  command-line options your boot loader passes to the kernel.
+
 config EXTRA_TARGETS
 	string "Additional default image types"
 	help