env/ext4.c: allow loading from an EXT4 partition on the MMC boot device

This parallels what I added for FAT in commit 6731bef6, allowing the
environment to be found in a specific partition on the device that the
board's mmc_get_env_dev() returns. On the Banana Pi R2 that means the
device that U-Boot was loaded from; either the internal eMMC or an SD
card.
Signed-off-by: NDavid Woodhouse <dwmw2@infradead.org>

env/Kconfig

@@ -477,6 +477,10 @@ config ENV_EXT4_DEVICE_AND_PART
 	                   If none, first valid partition in device D. If no
 	                   partition table then means device D.
 
+	  If ENV_EXT4_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_EXT4_FILE
 	string "Name of the EXT4 file to use for the environment"
 	depends on ENV_IS_IN_EXT4

env/ext4.c

@@ -41,7 +41,21 @@ __weak const char *env_ext4_get_intf(void)
 
 __weak const char *env_ext4_get_dev_part(void)
 {
+#ifdef CONFIG_MMC
+	static char *part_str;
+
+	if (!part_str) {
+		part_str = CONFIG_ENV_EXT4_DEVICE_AND_PART;
+		if (!strcmp(CONFIG_ENV_EXT4_INTERFACE, "mmc") && part_str[0] == ':') {
+			part_str = "0" CONFIG_ENV_EXT4_DEVICE_AND_PART;
+			part_str[0] += mmc_get_env_dev();
+		}
+	}
+
+	return part_str;
+#else
 	return (const char *)CONFIG_ENV_EXT4_DEVICE_AND_PART;
+#endif
 }
 
 static int env_ext4_save_buffer(env_t *env_new)