ilding of the external kernel modules:
You can rebuild the packages using the @module-rebuild set:
emerge --ask @module-rebuild
Solving build problems
When experiencing build problems while rebuilding the current kernel, it might help to sanitize the kernel sources. Make sure to backup the .config file first, as the operation will remove it. Make sure not to use a .bak or ~ suffix as backup as make distclean will clean those up as well.
cp .config /usr/src/kernel_config_bk
mv /usr/src/kernel_config_bk .config
Removing old kernels
See the kernel removal article.