kernel.mk: Correctly allow triplet when setting custom toolchain

Earlier patchsets were a bust. Dunno what I was thinking.

Patch inspired from: https://github.com/aokpx/platform_build/commit/7c65e1f0081352306e44421423d50fb4e80826e5

Change-Id: I2ee0b2cf8844e77cbb8d73a8faefd5b904c4b236