Allow configuration of gcc version used for build

You can override the default version by setting the value of
TARGET_GCC_VERSION in the BoardConfig.mk file. For example,
to select version 4.6, add the following:

TARGET_GCC_VERSION := 4.6

Change-Id: I10afb47b18c45e2b913b8af8f34cb98e5e96cbf2

Conflicts:
	core/config.mk