swupdate: ensure TARGET_CC is used for compiling/linking

Fixes:
http://autobuild.buildroot.net/results/e302d0edb59ff7617b5f2d21f06eb65ae04981fe
http://autobuild.buildroot.net/results/dbb69acadc20b4bb559311348eca276c1e6343f7



Swupdate uses $CROSS-cc instead of $CROSS-gcc, which is not available in all
external toolchains, and use CC for linking. Ensure TARGET_CC is used for both.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Cc: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>