rpm: ignore CMakeCache.txt in dist and relax BuildRequires on cross build

CPack takes the source dir as is, so if it was used to build something
it will incorrectly grab the temporary CMakeCache file and cmake will
complain during rpmbuild later on.

The BuildRequires should be a separate patch but logic behind the change
is that the dependencies need to be installed in the sysroot, and
rpmbuild cannot test this, so just move them all to only enforce
BuildRequires for native build.

And while we are here, also add a new kernel_dir specfile option.

Change-Id: Ie67932798f632e6d307f8ead93bdbe043e6e8898

.gitignore

@@ -33,3 +33,8 @@ executer/user/libmcexec.a
 executer/user/libldump2mcdump.so
 executer/user/eclair
 tools/mcstat/mcstat
+/_CPack_Packages
+/CPackSourceConfig.cmake
+CPackConfig.cmake
+/build
+mckernel-*.tar.gz