toolchain/external: avr32 is no more

Slightly reword a comment to no longer mention avr32.

This part dealing with sysroot detection will have to be reworked, now
that we got rid of avr32: we can now require a fully sysroot-aware
toolchain, i.e. at least gcc-4.4.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
CC: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>