embestmx6boards: Use PARTUUID to specify the rootfs location

Currently the rootfs location is passed via mmcblk number and the
problem with this approach is that the mmcblk number for the eMMC
changes depending on the kernel version.

In order to avoid such issue, use UUID method to specify the rootfs
location.

Also add CONFIG_BOOTCOMMAND to run finduuid function and distro_bootcmd.

This change was made based on U-Boot commit:

  - ca4f338eSigned-off-by: NFabio Berton <fabio.berton@ossystems.com.br>
Signed-off-by: NOtavio Salvador <otavio@ossystems.com.br>
Reviewed-by: NFabio Estevam <fabio.estevam@nxp.com>

include/configs/embestmx6boards.h

@@ -15,7 +15,6 @@
 
 #define CONFIG_MXC_UART_BASE		UART2_BASE
 #define CONSOLE_DEV		"ttymxc1"
-#define CONFIG_MMCROOT			"/dev/mmcblk1p2"
 
 #define PHYS_SDRAM_SIZE		(1u * 1024 * 1024 * 1024)
 
@@ -140,6 +139,10 @@
 	func(PXE, pxe, na) \
 	func(DHCP, dhcp, na)
 
+#define CONFIG_BOOTCOMMAND \
+	"run finduuid; " \
+	"run distro_bootcmd"
+
 #include <config_distro_bootcmd.h>
 
 #define CONSOLE_STDIN_SETTINGS \
@@ -157,6 +160,7 @@
 	CONSOLE_ENV_SETTINGS \
 	MEM_LAYOUT_ENV_SETTINGS \
 	"fdtfile=" CONFIG_FDTFILE "\0" \
+	"finduuid=part uuid mmc 0:1 uuid\0" \
 	BOOTENV
 
 #endif                         /* __RIOTBOARD_CONFIG_H */