build: Add support for TARGET_EXTRA_C{,PP}FLAGS

Add support for setting TARGET_EXTRA_C{,PP}FLAGS in board configs.
This can be used to set compiler flags for a board that break other
boards, such as -march=cortex-a9 or -DUSE_CUSTOM_EXTENSIONS_FOR_XYZ

Change-Id: I0efd4e37578bf6d239bf56dfeeaaed44840ee3af
Signed-off-by: Bernhard Rosenkraenzer <Bernhard.Rosenkranzer@linaro.org>

Other use cases include disabling flags that might break things on some devices
using the -fno function

PS1 - Added a check for ARM in core/config.mk
PS2 - Remove check for ARM and check if defined of not
PS3 - Fix backwards ifdef's and ifndef's
PS4 - Rework logic.  If TARGET_EXTRA_CFLAGS is not defined do nothing
PS5 - Fix adding wrong flags
PS6 - Fix one more small error.  Good to go now

Signed-off-by: Paul Beeler <sparksco@gmail.com>

Conflicts:
	core/combo/TARGET_linux-arm.mk