stmhal: Add home/end cursor support in readline.

Home/end work in picocom and screen (different codes in those 2
programs).  Also, CTRL-A (for non-empty liny) and CTRL-E act as
home/end.

stmhal/readline.c

@@ -31,6 +31,7 @@ int readline(vstr_t *line, const char *prompt) {
     stdout_tx_str(prompt);
     int orig_line_len = line->len;
     int escape_seq = 0;
+    char escape_seq_buf[1] = {0};
     int hist_cur = -1;
     int cursor_pos = orig_line_len;
     for (;;) {
@@ -43,6 +44,12 @@ int readline(vstr_t *line, const char *prompt) {
             if (VCP_CHAR_CTRL_A <= c && c <= VCP_CHAR_CTRL_D && vstr_len(line) == orig_line_len) {
                 // control character with empty line
                 return c;
+            } else if (c == VCP_CHAR_CTRL_A) {
+                // CTRL-A with non-empty line is go-to-start-of-line
+                redraw_step_back = cursor_pos - orig_line_len;
+            } else if (c == VCP_CHAR_CTRL_E) {
+                // CTRL-E is go-to-end-of-line
+                redraw_step_forward = line->len - cursor_pos;
             } else if (c == '\r') {
                 // newline
                 stdout_tx_str("\r\n");
@@ -80,6 +87,10 @@ int readline(vstr_t *line, const char *prompt) {
                 escape_seq = 0;
             }
         } else if (escape_seq == 2) {
+            if ('0' <= c && c <= '9') {
+                escape_seq = 3;
+                escape_seq_buf[0] = c;
+            } else {
                 escape_seq = 0;
                 if (c == 'A') {
                     // up arrow
@@ -120,6 +131,18 @@ int readline(vstr_t *line, const char *prompt) {
                         redraw_step_back = 1;
                     }
                 }
+            }
+        } else if (escape_seq == 3) {
+            escape_seq = 0;
+            if (c == '~') {
+                if (escape_seq_buf[0] == '1' || escape_seq_buf[0] == '7') {
+                    // home key
+                    redraw_step_back = cursor_pos - orig_line_len;
+                } else if (escape_seq_buf[0] == '4' || escape_seq_buf[0] == '8') {
+                    // end key
+                    redraw_step_forward = line->len - cursor_pos;
+                }
+            }
         } else {
             escape_seq = 0;
         }

stmhal/usb.h

@@ -3,6 +3,7 @@
 #define VCP_CHAR_CTRL_B (2)
 #define VCP_CHAR_CTRL_C (3)
 #define VCP_CHAR_CTRL_D (4)
+#define VCP_CHAR_CTRL_E (5)
 
 typedef enum {
     USB_DEVICE_MODE_CDC_MSC,