env/fat.c: allow loading from a FAT partition on the MMC boot device

I don't want to have to specify the device; only the partition.

This allows me to use the same image on internal eMMC or SD card for
Banana Pi R2, and it finds its own environment either way.
Signed-off-by: NDavid Woodhouse <dwmw2@infradead.org>
[trini: Add #if/#else/#endif logic around CONFIG_SYS_MMC_ENV_DEV usage,
        whitespace changes]
Signed-off-by: NTom Rini <trini@konsulko.com>

env/Kconfig

@@ -434,6 +434,10 @@ config ENV_FAT_DEVICE_AND_PART
 	                   If none, first valid partition in device D. If no
 	                   partition table then means device D.
 
+	  If ENV_FAT_INTERFACE is set to "mmc" then device 'D' can be omitted,
+	  leaving the string starting with a colon, and the boot device will
+	  be used.
+
 config ENV_FAT_FILE
 	string "Name of the FAT file to use for the environment"
 	depends on ENV_IS_IN_FAT

env/fat.c

@@ -29,6 +29,34 @@
 # define LOADENV
 #endif
 
+__weak int mmc_get_env_dev(void)
+{
+#ifdef CONFIG_SYS_MMC_ENV_DEV
+	return CONFIG_SYS_MMC_ENV_DEV;
+#else
+	return 0;
+#endif
+}
+
+static char *env_fat_device_and_part(void)
+{
+#ifdef CONFIG_MMC
+	static char *part_str;
+
+	if (!part_str) {
+		part_str = CONFIG_ENV_FAT_DEVICE_AND_PART;
+		if (!strcmp(CONFIG_ENV_FAT_INTERFACE, "mmc") && part_str[0] == ':') {
+			part_str = "0" CONFIG_ENV_FAT_DEVICE_AND_PART;
+			part_str[0] += mmc_get_env_dev();
+		}
+	}
+
+	return part_str;
+#else
+	return CONFIG_ENV_FAT_DEVICE_AND_PART;
+#endif
+}
+
 static int env_fat_save(void)
 {
 	env_t __aligned(ARCH_DMA_MINALIGN) env_new;
@@ -43,7 +71,7 @@ static int env_fat_save(void)
 		return err;
 
 	part = blk_get_device_part_str(CONFIG_ENV_FAT_INTERFACE,
-					CONFIG_ENV_FAT_DEVICE_AND_PART,
+					env_fat_device_and_part(),
 					&dev_desc, &info, 1);
 	if (part < 0)
 		return 1;
@@ -89,7 +117,7 @@ static int env_fat_load(void)
 #endif
 
 	part = blk_get_device_part_str(CONFIG_ENV_FAT_INTERFACE,
-					CONFIG_ENV_FAT_DEVICE_AND_PART,
+					env_fat_device_and_part(),
 					&dev_desc, &info, 1);
 	if (part < 0)
 		goto err_env_relocate;