Please mark Cache_Read_Enable_New as weak

[rab]

Hi,

rBoot is an open source bootloader with more features than the standard sdk bootloader (https://github.com/raburton/esp8266/tree/master/rboot). To enable memory mapping of different parts of the flash it needs to override Cache_Read_Enable_New from the sdk. Could you please mark this function as weak so that it can be easily overridden? It is possible without, but requires a modification to libmain with the objcopy tool. It would be much nicer if this was just marked as weak in the sdk, and would do no harm to anyone who wasn't overriding it - it would continue to work just the same.

Thanks,
Richard.