cdrkit: fix TARGET_CC/TARGET_CFLAGS for CMake

Since the reorganization of the variables in package/Makefile.in,
TARGET_CC and TARGET_CXX now directly contain the --sysroot= option in
addition to the compiler path. This is due to some ./configure scripts
using just $(TARGET_CC) for some tests instead of $(TARGET_CC)
$(TARGET_CFLAGS).

However, in the case of CMake, this fails as CMake really only wants
the path of the compiler in its CMAKE_C_COMPILER and
CMAKE_CXX_COMPILER variables. So here, we recompute proper values for
CMake by removing the --sysroot option from the compiler variables and
re-adding it to the flags variables.

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