machine: freedom-u540: Add support for booting from SD card

Modify freedom-u540 machine to boot off of sd card by default.
Note that there is a bug with the U540's SPI controller that
makes this painfully slow.

Signed-off-by: Moritz Fischer <moritz.fischer@ettus.com>

recipes-kernel/linux/files/freedom-u540/0001-risc-v-Fix-issue-ignoring-CONFIG_CMDLINE_OVERRIDE.patch

@@ -0,0 +1,42 @@
+From dc58ccb30dd1152a2483f4a3d4e2476a7f0d54cf Mon Sep 17 00:00:00 2001
+From: Moritz Fischer <mdf@kernel.org>
+Date: Sat, 7 Apr 2018 22:01:54 +0000
+Subject: [PATCH] risc-v: Fix issue ignoring CONFIG_CMDLINE_OVERRIDE
+
+Fix issue with ignoring CONFIG_CMDLINE_OVERRIDE.
+
+Upstream-status: Inapporopriate [Not correct fix]
+Signed-off-by: Moritz Fischer <mdf@kernel.org>
+---
+ arch/riscv/kernel/setup.c | 7 +++++++
+ 1 file changed, 7 insertions(+)
+
+diff --git a/arch/riscv/kernel/setup.c b/arch/riscv/kernel/setup.c
+index a1d585387f60..727caa5f5f42 100644
+--- a/arch/riscv/kernel/setup.c
++++ b/arch/riscv/kernel/setup.c
+@@ -39,6 +39,8 @@
+ #include <asm/tlbflush.h>
+ #include <asm/thread_info.h>
+ 
++static char default_command_line[COMMAND_LINE_SIZE] __initdata = CONFIG_CMDLINE;
++
+ #ifdef CONFIG_EARLY_PRINTK
+ static void sbi_console_write(struct console *co, const char *buf,
+ 			      unsigned int n)
+@@ -222,7 +224,12 @@ void __init setup_arch(char **cmdline_p)
+                register_console(early_console);
+        }
+ #endif
++
++#if defined(CONFIG_CMDLINE_OVERRIDE)
++	*cmdline_p = default_command_line;
++#else
+ 	*cmdline_p = boot_command_line;
++#endif
+ 
+ 	parse_early_param();
+ 
+-- 
+2.14.3
+