esp8266/main: Bump heap size to 28K.

This is kind of compensation for 4K FatFs buffer size which is eaten away
from it on FS mount. This should still leave enough of networking ("OS")
heap.

esp8266/main.c

@@ -38,7 +38,7 @@
 #include "gccollect.h"
 #include "user_interface.h"
 
-STATIC char heap[24 * 1024];
+STATIC char heap[28 * 1024];
 
 STATIC void mp_reset(void) {
     mp_stack_set_top((void*)0x40000000);