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_nand.c b/common/cmd_nand.c
index 04ab0f1..a84f7dc 100644
--- a/common/cmd_nand.c
+++ b/common/cmd_nand.c
@@ -605,22 +605,16 @@
 		opts.spread = spread;
 
 		if (scrub) {
-			if (!scrub_yes)
-				puts(scrub_warn);
-
-			if (scrub_yes)
+			if (scrub_yes) {
 				opts.scrub = 1;
-			else if (getc() == 'y') {
-				puts("y");
-				if (getc() == '\r')
+			} else {
+				puts(scrub_warn);
+				if (confirm_yesno()) {
 					opts.scrub = 1;
-				else {
+				} else {
 					puts("scrub aborted\n");
 					return 1;
 				}
-			} else {
-				puts("scrub aborted\n");
-				return 1;
 			}
 		}
 		ret = nand_erase_opts(nand, &opts);