configure: document ignored switches

Autostuff tradiotionnally have some switches to set host, build and
target systems, static or shared libs, and a bunch of others...

In the crosstool-NG case, they do not matter, as crosstool-NG is
just a set of scripts (there are C files, but they are compiled
after installation, at runtime, so it does not count).

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 # Blackfin specific configuration file

 ## select ARCH_SUPPORTS_32

 ## select ARCH_DEFAULT_32

 ## select ARCH_DEFAULT_LE

 ## select ARCH_SUPPORT_ARCH

 ## select ARCH_SUPPORT_CPU

 ## select ARCH_SUPPORT_TUNE

 ## select ARCH_SUPPORT_FPU

 ##

 ## help The Blackfin architecture