build/core: Remove useless warning when using OpenJDK

Rebased.

We provide and use a custom prebuilt OpenJDK 7 by default, so remove
this useless warning.

Change-Id: I61578c6b966bc72b328404c4a74e585a4f781d93

Conflicts:

	core/main.mk
build: Set OpenJDK as default java compiler

Rebased.

Change-Id: I8494669517169b8377f8ffaaa1cbe7356f38a139

Conflicts:

	envsetup.sh

Conflicts:

	envsetup.sh
kernel.mk: Allow triplet when seeting custom toolchain

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

Change-Id: Idcf3829f056ca1454c2478182df706a1ba04bea5