lib/json: Fix build warnings (-Wpointer-sign) with LLVM/icx

Move all characters to "char" type: no implicit conversions between
"unsigned char", "u8_t", etc.

Tested with ISSM 2016.2.085.

Jira: ZEP-2159
Signed-off-by: Leandro Pereira <leandro.pereira@intel.com>
diff --git a/lib/json/json.c b/lib/json/json.c
index 41faaf7..4d4ec6e 100644
--- a/lib/json/json.c
+++ b/lib/json/json.c
@@ -24,9 +24,9 @@
 
 struct lexer {
 	void *(*state)(struct lexer *lexer);
-	unsigned char *start;
-	unsigned char *pos;
-	unsigned char *end;
+	char *start;
+	char *pos;
+	char *end;
 	struct token token;
 };
 
@@ -76,7 +76,7 @@
 	lexer->start = lexer->pos;
 }
 
-static unsigned char next(struct lexer *lexer)
+static char next(struct lexer *lexer)
 {
 	if (lexer->pos >= lexer->end) {
 		lexer->pos = lexer->end + 1;
@@ -97,9 +97,9 @@
 	lexer->pos--;
 }
 
-static unsigned char peek(struct lexer *lexer)
+static char peek(struct lexer *lexer)
 {
-	unsigned char chr = next(lexer);
+	char chr = next(lexer);
 
 	backup(lexer);
 
@@ -111,7 +111,7 @@
 	ignore(lexer);
 
 	while (true) {
-		unsigned char chr = next(lexer);
+		char chr = next(lexer);
 
 		if (chr == '\0') {
 			emit(lexer, JSON_TOK_ERROR);
@@ -216,7 +216,7 @@
 static void *lexer_number(struct lexer *lexer)
 {
 	while (true) {
-		unsigned char chr = next(lexer);
+		char chr = next(lexer);
 
 		if (isdigit(chr) || chr == '.') {
 			continue;
@@ -232,7 +232,7 @@
 static void *lexer_json(struct lexer *lexer)
 {
 	while (true) {
-		unsigned char chr = next(lexer);
+		char chr = next(lexer);
 
 		switch (chr) {
 		case '\0':
@@ -596,7 +596,7 @@
 	return obj_parse(&obj, descr, descr_len, val);
 }
 
-static u8_t escape_as(u8_t chr)
+static char escape_as(char chr)
 {
 	switch (chr) {
 	case '"':
@@ -626,10 +626,10 @@
 	int ret = 0;
 
 	for (cur = str; ret == 0 && *cur; cur++) {
-		u8_t escaped = escape_as(*cur);
+		char escaped = escape_as(*cur);
 
 		if (escaped) {
-			u8_t bytes[2] = { '\\', escaped };
+			char bytes[2] = { '\\', escaped };
 
 			ret = append_bytes(bytes, 2, data);
 		} else {
@@ -864,7 +864,7 @@
 	size_t size;
 };
 
-static int append_bytes_to_buf(const u8_t *bytes, size_t len, void *data)
+static int append_bytes_to_buf(const char *bytes, size_t len, void *data)
 {
 	struct appender *appender = data;
 
@@ -888,7 +888,7 @@
 			       &appender);
 }
 
-static int measure_bytes(const u8_t *bytes, size_t len, void *data)
+static int measure_bytes(const char *bytes, size_t len, void *data)
 {
 	ssize_t *total = data;