toolchain-scripts.bbclass: Export KCFLAGS to ensure sysroot is provided

toolchain-scripts.bbclass: Export KCFLAGS to ensure sysroot is provided

When building the U-Boot the lack of a proper sysroot can trigger
following error:

,----
| arm-poky-linux-gnueabi-ld.bfd: cannot find -lgcc
| make[2]: *** [examples/standalone/hello_world] Error 1
| make[1]: *** [examples/standalone] Error 2
| make: *** [examples] Error 2
`----

Guillaume Fournier has posted a very complete analysis of the
problem[1].

1. https://lists.yoctoproject.org/pipermail/meta-freescale/2014-November/011270.html

The use of KCFLAGS makes the build of U-Boot work out of box, now that
it uses the Linux kernel build system.

Reported-by: Guillaume Fournier <gfournier@brioconcept.com>
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Ross Burton <ross.burton@intel.com>