env: Move get/set_default_env() to env.h

Move these functions to the new header file and rename set_default_env()
to env_set_default() so that it has a consistent env_ prefix.

Acked-by: Joe Hershberger <joe.hershberger@ni.com>
Signed-off-by: Simon Glass <sjg@chromium.org>

board/phytec/pcm052/pcm052.c

@@ -377,7 +377,7 @@ int board_late_init(void)
 	if ((reg & SRC_SBMR1_BOOTCFG1_SDMMC) &&
 	    !(reg & SRC_SBMR1_BOOTCFG1_MMC)) {
 		printf("------ SD card boot -------\n");
-		set_default_env("!LVFBootloader", 0);
+		env_set_default("!LVFBootloader", 0);
 		env_set("bootcmd",
 			"run prepare_install_bk4r1_envs; run install_bk4r1rs");
 	}