If the user asks for "kernel sources" to be installed, extract the

SRC_BASE package (src/[A-Z]*) as well as SRC_SYS (src/sys/*).  This
allows users who only install the kernel source code to use the
modern "make buildkernel" approach.

Discussed with:	re (scottl, kensmith)
MFC after:	3 days

usr.sbin/sysinstall/dist.c

@@ -254,7 +254,7 @@ distSetKernDeveloper(dialogMenuItem *self)
 
     distReset(NULL);
     Dists = _DIST_DEVELOPER;
-    SrcDists = DIST_SRC_SYS;
+    SrcDists = DIST_SRC_SYS | DIST_SRC_BASE;
     KernelDists = selectKernel();
     i = distMaybeSetPorts(self);
     distVerifyFlags();