Documentation: quotes around TARGET_CC command.

When ccache is enabled, TARGET_CC is "ccache gcc". Without quotes,
when assigning CC=$(TARGET_CC), only "ccache" gets assigned to CC.

Signed-off-by: Shawn J. Goff <shawn7400@gmail.com>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>