use extend, otherwise append from qemu is ignored

target/sh/kernel/qemu-sh

@@ -13,7 +13,7 @@ CONFIG_SH_RTS7751R2D=y
 CONFIG_RTS7751R2D_PLUS=y
 CONFIG_SH_TIMER_TMU=y
 CONFIG_SH_INTC=y
-CONFIG_CMDLINE_OVERWRITE=y
+CONFIG_CMDLINE_EXTEND=y
 CONFIG_PCI=y
 CONFIG_PCI_DOMAINS=y
 CONFIG_MII=y