Fix linker script for 512k(no SPIFFS) variant (esp8266#966)

thanks @sven337

Author: igrr

tools/sdk/ld/eagle.flash.512k0.ld

@@ -10,8 +10,8 @@ MEMORY
   irom0_0_seg :                         org = 0x40201010, len = 0x79ff0
 }
 
-PROVIDE ( _SPIFFS_start = 0 );
-PROVIDE ( _SPIFFS_end = 0 );
+PROVIDE ( _SPIFFS_start = 0x4027B000 );
+PROVIDE ( _SPIFFS_end = 0x4027B000 );
 PROVIDE ( _SPIFFS_page = 0 );
 PROVIDE ( _SPIFFS_block = 0 );