sunxi: Create helper function veryfing valid boot signature on MMC

This patch extracts checking for valid SD card "eGON.BT0" signature from
`board_mmc_init` into function `sunxi_mmc_has_egon_boot_signature`.

Buffer for mmc sector is allocated and freed at runtime. `panic` is
triggered on malloc failure.
Signed-off-by: NDaniel Kochmański <dkochmanski@turtle-solutions.eu>
CC: Roy Spliet <r.spliet@ultimaker.com>
Cc: Ian Campbell <ijc@hellion.org.uk>
[hdegoede@redhat.com: Small bugfix to make it work for devs other then mmc0]
Acked-by: NHans de Goede <hdegoede@redhat.com>
Signed-off-by: NHans de Goede <hdegoede@redhat.com>

arch/arm/include/asm/arch-sunxi/mmc.h

@@ -127,4 +127,5 @@ struct sunxi_mmc {
 #define SUNXI_MMC_COMMON_RESET			(1 << 18)
 
 struct mmc *sunxi_mmc_init(int sdc_no);
+int sunxi_mmc_has_egon_boot_signature(struct mmc *mmc);
 #endif /* _SUNXI_MMC_H */

board/sunxi/board.c

@@ -299,12 +299,8 @@ int board_mmc_init(bd_t *bis)
 	 * Both mmc0 and mmc2 are bootable, figure out where we're booting
 	 * from. Try mmc0 first, just like the brom does.
 	 */
-	if (mmc_getcd(mmc0) && mmc_init(mmc0) == 0 &&
-	    mmc0->block_dev.block_read(0, 16, 1, buf) == 1) {
-		buf[12] = 0;
-		if (strcmp(&buf[4], "eGON.BT0") == 0)
-			return 0;
-	}
+	if (sunxi_mmc_has_egon_boot_signature(mmc0))
+		return 0;
 
 	/* no bootable card in mmc0, so we must be booting from mmc2, swap */
 	mmc0->block_dev.dev = 1;

drivers/mmc/sunxi_mmc.c

@@ -433,6 +433,23 @@ static int sunxi_mmc_getcd(struct mmc *mmc)
 	return !gpio_get_value(cd_pin);
 }
 
+int sunxi_mmc_has_egon_boot_signature(struct mmc *mmc)
+{
+	char *buf = malloc(512);
+	int valid_signature = 0;
+
+	if (buf == NULL)
+		panic("Failed to allocate memory\n");
+
+	if (mmc_getcd(mmc) && mmc_init(mmc) == 0 &&
+	    mmc->block_dev.block_read(mmc->block_dev.dev, 16, 1, buf) == 1 &&
+	    strncmp(&buf[4], "eGON.BT0", 8) == 0)
+		valid_signature = 1;
+
+	free(buf);
+	return valid_signature;
+}
+
 static const struct mmc_ops sunxi_mmc_ops = {
 	.send_cmd	= sunxi_mmc_send_cmd,
 	.set_ios	= sunxi_mmc_set_ios,