RISC-V: Add early startup code

On entry, we need to save the system table pointer as well as our image
handle. Add an early startup file that saves them and then brings us
into our main function.

Signed-off-by: Alexander Graf <agraf@suse.de>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

grub-core/kern/riscv/efi/startup.S

@@ -0,0 +1,48 @@
+/*
+ *  GRUB  --  GRand Unified Bootloader
+ *  Copyright (C) 2018  Free Software Foundation, Inc.
+ *
+ *  GRUB is free software: you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License as published by
+ *  the Free Software Foundation, either version 3 of the License, or
+ *  (at your option) any later version.
+ *
+ *  GRUB is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU General Public License for more details.
+ *
+ *  You should have received a copy of the GNU General Public License
+ *  along with GRUB.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include <grub/symbol.h>
+
+#if __riscv_xlen == 64
+#define sl		sd
+#define ll		ld
+#else
+#define sl		sw
+#define ll		lw
+#endif
+
+
+	.file 	"startup.S"
+	.text
+FUNCTION(_start)
+	/*
+	 *  EFI_SYSTEM_TABLE and EFI_HANDLE are passed in a1/a0.
+	 */
+
+	ll	a2, efi_image_handle_val
+	sl	a0, 0(a2)
+	ll	a2, efi_system_table_val
+	sl	a1, 0(a2)
+	ll	a2, grub_main_val
+	jr	a2
+grub_main_val:
+	.quad	EXT_C(grub_main)
+efi_system_table_val:
+	.quad	EXT_C(grub_efi_system_table)
+efi_image_handle_val:
+	.quad	EXT_C(grub_efi_image_handle)