Bring back the definition of strcpy

494320b removed strcpy() from rtl837x_flash.c but left its declaration
in rtl837x_common.h. Since then the linker has taken strcpy from the
SDCC library instead, which expects the destination as a generic pointer
in DPL, DPH and B. The callers pass an __xdata pointer and leave B as it
is, so the copy lands wherever B happens to point. boot.c only gets away
with it because B is zero there; a call with B set, as from the ChaCha20
test in #207, writes into internal RAM and stops the 8051.

rtlplayground.c

@@ -410,6 +410,16 @@ uint16_t strlen_x(__xdata const char *s)
 }
 
 
+uint16_t strcpy(__xdata uint8_t *dst, const char *s)
+{
+	__xdata uint8_t *b = dst;
+	while (*s)
+		*dst++ = *s++;
+	*dst = 0;
+	return dst - b;
+}
+
+
 char strcmp(__xdata const uint8_t *a, __code const uint8_t *b)
 {
 	uint8_t i = 0;