Bootrom: Enable serial number from flash

* Add `.ramfunc` section to bootrom loader script
* exclude spiffs functionality from flashmem.h/flashmem.c
   (allows bootrom to use flashmem)
* hide unused tick.h / flashmem.h functions from bootrom
   (not technically necessary; see comments)
* bootrom: add source files, include path, and defines when
  `PLATFORM_DEFS` defines `WITH_FLASH`
* Define `AS_BOOTROM` to indicate code is building for bootrom

bootrom/ldscript-flash

@@ -53,6 +53,7 @@ SECTIONS
         *(.rodata.*)
         *(.data)
         *(.data.*)
+        *(.ramfunc)
         . = ALIGN(4);
     } >ram AT>bootphase2 :phase2