tftpd: disallow on nios with pre-6.x toolchains

tftpd: disallow on nios with pre-6.x toolchains

Fixes:
http://autobuild.buildroot.org/results/349/349bc8668614b0f2c970ff33f25216e708ecdb46/
http://autobuild.buildroot.org/results/264/26486e88211d01ee49ad786e9a27a67b849ad1ab/

Older toolchains contain an issue with the linker script causing failures when linking tftpd:

tftpd.c:(.text.startup+0x5c): warning: Unable to reach (null) (at 0x00000000)
from the global pointer (at 0x00011a80) because the offset (-72320) is out of
the allowed range, -32678 to 32767.

This is a known issue which has been fixed from 6.x, so disallow on older
toolchains:

https://www.altera.com/support/support-resources/knowledge-base/solutions/rd02132012_291.html

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>