toolchain: external 3.17 headers typo fix

Commit 2d312b7b had a typo
"." instead of "_" in BR2_TOOLCHAIN_HEADERS_AT_LEAST_3.17.
This made selecting 3.17 as custom external headers version
impossible.

Signed-off-by: Karoly Kasza <kaszak@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>