sed: fix compilation with external toolchains

Since CC is already passed in TARGET_CONFIGURE_OPTS at ./configure
time, there's no point in overriding it with a wrong value at
compilation time. This breaks external toolchain builds since
CC=$(TARGET_CC) is wrong because it excludes the mandatory --sysroot
option.

Fixes bug #525

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>