cbootimage: disable on musl

cbootimage: disable on musl

cbootimage uses non-standard u_intXX_t types instead of the C99 types
uintXX_t types, which prevents it from building with the musl C library.

A patch to fix this has been submitted upstream, but since there are
many many occurences of such types, the patch is a bit large to carry in
Buildroot, especially compared to the fact that having this package
building against musl is probably not a top priority.

So for now, simply disable this package on musl, until upstream fixes
the issue and releases a new version.

Fixes:

  http://autobuild.buildroot.net/results/61bdfb7e0ff9628190d9eb86e40c4c90e768b8e2

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