linux26: do not set CFLAGS_KERNEL to TARGET_CFLAGS

Using TARGET_CFLAGS breaks the kernel build for the arm platform. The
result is a kernel that can not be booted. So simply do not fiddle with
the CFLAGS on a kernel build and the kernel build system will do the
right thing (TM).

[Peter: do the same thing for the standard kernel build]

Signed-off-by: Sven Neumann <s.neumann@raumfeld.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>