Allow config of gcc version used/Make gcc 4.7, -O3 defaults

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

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Custom ToolchainModHowto
* use gcc 4.6 add TARGET_GCC_VERSION := 4.6 to your board config
* to use O2 add TARGET_USE_O2 := true to enable -O2

Credits to PA for the Idea and layout

Conflicts:
	core/combo/TARGET_linux-arm.mk
	core/combo/select.mk

Conflicts:
	core/combo/TARGET_linux-arm.mk
	core/combo/select.mk