debug/gdb: hide Linaro options by default

It can be quite confusing for a new-comer to find strange
version numbers for gdb, so hide the Linaro versions by
default.

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

 # Linux kernel options

 config KERNEL_linux

     select KERNEL_SUPPORTS_SHARED_LIBS

     help

       Build a toolchain targeting systems running Linux as a kernel.