env: Use getenv_yesno() more generally

Move the getenv_yesno() to env_common.c and change most checks for
'y' or 'n' to use this helper.

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>

arch/sparc/lib/board.c

@@ -284,8 +284,7 @@ void board_init_f(ulong bootflag)
 		 *
 		 * NOTE: Maybe we should add some WATCHDOG_RESET()? XXX
 		 */
-		s = getenv("flashchecksum");
-		if (s && (*s == 'y')) {
+		if (getenv_yesno("flashchecksum") == 1) {
 			printf("  CRC: %08lX",
 			       crc32(0, (const unsigned char *)CONFIG_SYS_FLASH_BASE,
 				     flash_size)