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`flash_<funcname>()` is implemented by `<funcname>_P()` provided by the AVR libc on AVR targets. Previously, the preprocessor was used to do the aliasing, but this causes issues with LLVM: The signatures of e.g. `printf_P()` expects `const char *`, whereas flash utils expects `FLASH_ATTR const char *`. For GCC this will just implicitly drop the `FLASH_ATTR`, while it requires an explicit cast for LLVM. To implement the explicit cast, `static inline` function wrappers where used instead where possible. But for the variadic functions (e.g. `printf(fmt, ...)`) the linker is used to provide the aliases, as there is no way to pass the variadic functions throw in C. The alternative would be to implement `flash_printf()` by calling `vprintf_P()`, but that increased ROM size quite a bit. Finally, a work around for a bug in Ubuntu's toolchain has been added: An unused function that calls to `printf_P()`, `fprintf_P()` and `snprintf_P()`. Since this function is garbage collected anyway, it has no impact on the generated ELF file.