esp8266/esp_mphal: Move most functions in esp_mphal.c from iRAM to iROM.

The ones that are moved out of iRAM should not need to be there, because
either they call functions in iROM (eg mp_hal_stdout_tx_str), or they are
only ever called from a function in iROM and not from an interrupt (eg
ets_esf_free_bufs).

This frees up about 800 bytes of iRAM.

ports/esp8266/boards/esp8266_common.ld

@@ -170,6 +170,7 @@ SECTIONS
         *modlwip.o(.literal* .text*)
         *modsocket.o(.literal* .text*)
         *modonewire.o(.literal* .text*)
+        *esp_mphal.o(.literal* .text*)
 
         /* we put as much rodata as possible in this section */
         /* note that only rodata accessed as a machine word is allowed here */