configs: lock kernel version for atngw100_defconfig

The 3.10.x kernels fail to build with the following message:

vsprintf.c:(.text+0x17e0): relocation truncated to fit: R_AVR32_16N_PCREL against symbol `_ctype' defined in .text section in lib/lib.a(ctype.o)

While this is most likely a toolchain issue, but since AVR32 isn't a
well-maintained architecture, there's not much we can do.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>