flash: prefix error codes with FL_

flash: prefix error codes with FL_

Prefix the flash status codes (ERR_*) with FL_ in order to avoid clashes
with third-party libraries. Case in point: including the lwIP library
header file <lwip/err.h> which defines err_enum_t as an enum with values
being ERR_*.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reviewed-by: Peter Robinson <pbrobinson@gmail.com>