grub-install: Check for arm-efi as a default target

Much like on x86, we can work out if the system is running on top
of EFI firmware. If so, return "arm-efi". If not, fall back to
"arm-uboot" as previously.

Heavily inspired by the existing code for x86.

Signed-off-by: Steve McIntyre <93sam@debian.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

grub-core/osdep/basic/platform.c

@@ -18,6 +18,12 @@
 
 #include <grub/util/install.h>
 
+const char *
+grub_install_get_default_arm_platform (void)
+{
+  return "arm-uboot";
+}
+
 const char *
 grub_install_get_default_x86_platform (void)
 { 

grub-core/osdep/linux/platform.c

@@ -97,6 +97,28 @@ read_platform_size (void)
   return ret;
 }
 
+const char *
+grub_install_get_default_arm_platform (void)
+{
+  /*
+   * On Linux, we need the efivars kernel modules. If no EFI is available this
+   * module just does nothing besides a small hello and if we detect efi we'll
+   * load it anyway later. So it should be safe to try to load it here.
+   */
+  grub_util_exec_redirect_all ((const char * []){ "modprobe", "efivars", NULL },
+			       NULL, NULL, "/dev/null");
+
+  grub_util_info ("Looking for /sys/firmware/efi ..");
+  if (is_not_empty_directory ("/sys/firmware/efi"))
+    {
+      grub_util_info ("...found");
+      return "arm-efi";
+    }
+
+  grub_util_info ("... not found");
+  return "arm-uboot";
+}
+
 const char *
 grub_install_get_default_x86_platform (void)
 {