Do not hardcode rootfs image name to allow automatic usage

This was a weird decision in the first place, I think it was to make it easier to compile the kernel directly, but it just causes issues in buildroot, which people should probably be using anyway...?

arch/arm/configs/nintendo3ds_defconfig

@@ -19,7 +19,6 @@ CONFIG_CHECKPOINT_RESTORE=y
 CONFIG_SCHED_AUTOGROUP=y
 CONFIG_RELAY=y
 CONFIG_BLK_DEV_INITRD=y
-CONFIG_INITRAMFS_SOURCE="rootfs.cpio.gz"
 # CONFIG_RD_BZIP2 is not set
 # CONFIG_RD_LZMA is not set
 # CONFIG_RD_XZ is not set
@@ -57,6 +56,7 @@ CONFIG_JUMP_LABEL=y
 # CONFIG_SECCOMP is not set
 # CONFIG_STACKPROTECTOR_STRONG is not set
 CONFIG_STRICT_KERNEL_RWX=y
+# CONFIG_GCC_PLUGINS is not set
 CONFIG_BLK_DEV_BSGLIB=y
 CONFIG_BLK_DEV_INTEGRITY=y
 CONFIG_BLK_DEV_THROTTLING=y
@@ -126,7 +126,6 @@ CONFIG_FB_MODE_HELPERS=y
 CONFIG_FB_SIMPLE=y
 CONFIG_LCD_CLASS_DEVICE=y
 CONFIG_BACKLIGHT_CLASS_DEVICE=y
-CONFIG_FRAMEBUFFER_CONSOLE=y
 CONFIG_FRAMEBUFFER_CONSOLE_ROTATION=y
 # CONFIG_HID is not set
 # CONFIG_USB_SUPPORT is not set
@@ -265,9 +264,7 @@ CONFIG_CRC7=y
 CONFIG_LIBCRC32C=y
 CONFIG_CRC8=y
 CONFIG_XZ_DEC=y
-CONFIG_FONTS=y
 CONFIG_FONT_8x8=y
-CONFIG_FONT_10x18=y
 CONFIG_PRINTK_TIME=y
 CONFIG_BOOT_PRINTK_DELAY=y
 CONFIG_DYNAMIC_DEBUG=y