common: Add support for environment file in EXT4.

This is an enhancement that permits storing the environment file on an
EXT4 partition such as the root filesystem.  It is based on the existing
FAT environment file code.
diff --git a/cmd/nvedit.c b/cmd/nvedit.c
index 5ae9d9d..9cf884e 100644
--- a/cmd/nvedit.c
+++ b/cmd/nvedit.c
@@ -45,6 +45,7 @@
 	!defined(CONFIG_ENV_IS_IN_DATAFLASH)	&& \
 	!defined(CONFIG_ENV_IS_IN_MMC)		&& \
 	!defined(CONFIG_ENV_IS_IN_FAT)		&& \
+	!defined(CONFIG_ENV_IS_IN_EXT4)		&& \
 	!defined(CONFIG_ENV_IS_IN_NAND)		&& \
 	!defined(CONFIG_ENV_IS_IN_NVRAM)	&& \
 	!defined(CONFIG_ENV_IS_IN_ONENAND)	&& \
@@ -53,7 +54,7 @@
 	!defined(CONFIG_ENV_IS_IN_UBI)		&& \
 	!defined(CONFIG_ENV_IS_NOWHERE)
 # error Define one of CONFIG_ENV_IS_IN_{EEPROM|FLASH|DATAFLASH|ONENAND|\
-SPI_FLASH|NVRAM|MMC|FAT|REMOTE|UBI} or CONFIG_ENV_IS_NOWHERE
+SPI_FLASH|NVRAM|MMC|FAT|EXT4|REMOTE|UBI} or CONFIG_ENV_IS_NOWHERE
 #endif
 
 /*