* 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