Add emmc_size envvar accessible from fastboot.

There are boards with several different eMMC sizes. We can easily
build gpt for each size and then flash the correct one to the
board. In order to identify this size, we can run fastboot command:

> fastboot getvar mmc_size

Change-Id: I32dac02b974753ed59f6db0bfbc5f7289f391f4c
diff --git a/board/freescale/imx8mq_phanbell/imx8mq_phanbell.c b/board/freescale/imx8mq_phanbell/imx8mq_phanbell.c
index 12a2732..d6818bd 100644
--- a/board/freescale/imx8mq_phanbell/imx8mq_phanbell.c
+++ b/board/freescale/imx8mq_phanbell/imx8mq_phanbell.c
@@ -247,16 +247,25 @@
 
 int board_late_init(void)
 {
+	char s[32] = {0};
+	int mmc_dev = mmc_get_env_dev();
+	struct mmc *mmc = find_mmc_device(mmc_dev);
+
+	if (mmc) {
+		snprintf(s, sizeof(s), "%llu", mmc->capacity_user);
+		env_set("fastboot.mmc_size", s);
+	}
+
 #ifdef CONFIG_ENV_VARS_UBOOT_RUNTIME_CONFIG
-	setenv("board_name", "Phanbell");
-	setenv("board_rev", "iMX8MQ");
+	env_set("board_name", "Phanbell");
+	env_set("board_rev", "iMX8MQ");
 #endif
 
 	static char bootdev[32];
 	static char bootcmd[128];
 	snprintf(bootdev, sizeof(bootdev), "%d", mmc_get_env_dev());
 	env_set("bootdev", bootdev);
-	snprintf(bootcmd, sizeof(bootcmd), "ext2load mmc %d:1 ${loadaddr} boot.scr; source; boota mmc0 boot_a", mmc_get_env_dev());
+	snprintf(bootcmd, sizeof(bootcmd), "ext2load mmc %d:1 ${loadaddr} boot.scr; source; boota mmc0 boot_a", mmc_dev);
 	env_set("bootcmd", bootcmd);
 
 #ifdef CONFIG_ENV_IS_IN_MMC