console: Move poor-man's atoi() into string.h

vtxprintf.c seems to have been written before string.h was as fleshed
out as it is today -- this patch removes some custom implementation of
stuff we now have globally. It also makes the skip_atoi() function
globally available, because I need it somewhere else, and while we maybe
don't want a huge fully-featured string parsing library in coreboot,
being able to parse an integer is occasionally useful.

Change-Id: Iecb2b970aecfc768540d2bf8b3023445f54853a4
Signed-off-by: Julius Werner <jwerner@chromium.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/32858
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Alex Thiessen <alex.thiessen.de+coreboot@gmail.com>

src/include/string.h

@@ -178,4 +178,19 @@ static inline int tolower(int c)
 		c -= 'A'-'a';
 	return c;
 }
+
+/*
+ * Parses an unsigned integer and moves the input pointer forward to the first
+ * character that's not a valid digit. s and *s must not be NULL. Result
+ * undefined if it overruns the return type size.
+ */
+static inline unsigned int skip_atoi(char **s)
+{
+	unsigned int i = 0;
+
+	while (isdigit(**s))
+		i = i*10 + *((*s)++) - '0';
+	return i;
+}
+
 #endif /* STRING_H */