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>
diff --git a/arch/powerpc/cpu/mpc85xx/mp.c b/arch/powerpc/cpu/mpc85xx/mp.c
index e1197ac..43d4836 100644
--- a/arch/powerpc/cpu/mpc85xx/mp.c
+++ b/arch/powerpc/cpu/mpc85xx/mp.c
@@ -46,10 +46,8 @@
  */
 int hold_cores_in_reset(int verbose)
 {
-	const char *s = getenv("mp_holdoff");
-
 	/* Default to no, overriden by 'y', 'yes', 'Y', 'Yes', or '1' */
-	if (s && (*s == 'y' || *s == 'Y' || *s == '1')) {
+	if (getenv_yesno("mp_holdoff") == 1) {
 		if (verbose) {
 			puts("Secondary cores are being held in reset.\n");
 			puts("See 'mp_holdoff' environment variable\n");