disk: efi: Correct backing up the MBR boot code

In commit e163a931af ("cmd: gpt: backup boot code before writing MBR")
there was added the procedure for storing old boot code when doing "gpt
write". But instead of storing just backup code, the whole MBR was
stored, and only specific fields were replaced further, keeping
everything else intact. That's obviously not what we want.

Fix the code to actually store only old boot code and zero out
everything else. This fixes next testing case:

    => mmc write $loadaddr 0x0 0x7b
    => gpt write mmc 1 $partitions

In case when $loadaddr address and further memory contains 0xff, the
board was bricked (ROM-code probably didn't like partition entries that
were clobbered with 0xff). With this patch applied, commands above don't
brick the board.

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Cc: Alejandro Hernandez <ajhernandez@ti.com>
Tested-by: Andy Shevchenko <andy.shevchenko@gmail.com>

disk/part_efi.c

@@ -350,8 +350,6 @@ static int set_protective_mbr(struct blk_desc *dev_desc)
 {
 	/* Setup the Protective MBR */
 	ALLOC_CACHE_ALIGN_BUFFER_PAD(legacy_mbr, p_mbr, 1, dev_desc->blksz);
-	memset(p_mbr, 0, sizeof(*p_mbr));
-
 	if (p_mbr == NULL) {
 		printf("%s: calloc failed!\n", __func__);
 		return -1;
@@ -363,6 +361,10 @@ static int set_protective_mbr(struct blk_desc *dev_desc)
 		return -1;
 	}
 
+	/* Clear all data in MBR except of backed up boot code */
+	memset((char *)p_mbr + MSDOS_MBR_BOOT_CODE_SIZE, 0, sizeof(*p_mbr) -
+			MSDOS_MBR_BOOT_CODE_SIZE);
+
 	/* Append signature */
 	p_mbr->signature = MSDOS_MBR_SIGNATURE;
 	p_mbr->partition_record[0].sys_ind = EFI_PMBR_OSTYPE_EFI_GPT;

include/part_efi.h

@@ -20,6 +20,7 @@
 #include <efi.h>
 
 #define MSDOS_MBR_SIGNATURE 0xAA55
+#define MSDOS_MBR_BOOT_CODE_SIZE 440
 #define EFI_PMBR_OSTYPE_EFI 0xEF
 #define EFI_PMBR_OSTYPE_EFI_GPT 0xEE
 
@@ -111,7 +112,7 @@ typedef struct _gpt_entry {
 } __packed gpt_entry;
 
 typedef struct _legacy_mbr {
-	u8 boot_code[440];
+	u8 boot_code[MSDOS_MBR_BOOT_CODE_SIZE];
 	__le32 unique_mbr_signature;
 	__le16 unknown;
 	struct partition partition_record[4];