Commit 2d66c780 ("cpuimx27 and mbimx27:
allow fine control of UART4 and SDHC2 usage") added the Kconfig symbol
MACH_EUKREA_CPUIMX27_USEUART4. But it forgot to prepend CONFIG_ to the
use of its macro. Add that prefix now.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>