Add the function 'confirm_yesno' for interactive

User's confirmation is asked in different commands. This commit adds a
function for such confirmation.

Acked-by: Pantelis Antoniou <panto@antoniou-consulting.com>
Signed-off-by: Pierre Aubert <p.aubert@staubli.com>
diff --git a/common/cmd_otp.c b/common/cmd_otp.c
index 67808aa..593bb8c 100644
--- a/common/cmd_otp.c
+++ b/common/cmd_otp.c
@@ -158,21 +158,9 @@
 			lowup(half + count - 1), page + (half + count - 1) / 2,
 			half + count
 		);
-
-		i = 0;
-		while (1) {
-			if (tstc()) {
-				const char exp_ans[] = "YES\r";
-				char c;
-				putc(c = getc());
-				if (exp_ans[i++] != c) {
-					printf(" Aborting\n");
-					return 1;
-				} else if (!exp_ans[i]) {
-					puts("\n");
-					break;
-				}
-			}
+		if (!confirm_yesno()) {
+			printf(" Aborting\n");
+			return 1;
 		}
 	}