toolchain/toolchain-buildroot: add comment about using virtual package infra

Commit "9a8ec9195c toolchain/toolchain-buildroot: migrate to virtual
package infrastructure" made the packages toolchain and
toolchain-buildroot to use the virtual package infra even they being
generic packages.
This works because on package/pkg-virtual.mk when a package do not
define neither _PROVIDES_ or _HAS_ symbols, only _IS_VIRTUAL is set to
YES and _VERSION and _SOURCE are set to empty before relaying the call
to inner-generic-package.

Add a comment explaining why the virtual package infra is used in these
cases.

Cc: Giulio Benetti <giulio.benetti@benettiengineering.com>
Cc: Romain Naour <romain.naour@gmail.com>
Cc: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
(cherry picked from commit abcecd737b198e8a1a48bf4533e5c2b8de8b8cf4)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

toolchain/toolchain-buildroot/toolchain-buildroot.mk

@@ -14,4 +14,7 @@ TOOLCHAIN_BUILDROOT_DEPENDENCIES = host-gcc-final
 
 TOOLCHAIN_BUILDROOT_ADD_TOOLCHAIN_DEPENDENCY = NO
 
+# Not really a virtual package, but we use the virtual package infra here so
+# both the build log and build directory look nicer (toolchain-buildroot-virtual
+# instead of toolchain-buildroot-undefined)
 $(eval $(virtual-package))

toolchain/toolchain/toolchain.mk

@@ -47,4 +47,7 @@ endef
 TOOLCHAIN_POST_INSTALL_TARGET_HOOKS += TOOLCHAIN_GLIBC_COPY_NSSWITCH_FILE
 endif
 
+# Not really a virtual package, but we use the virtual package infra here so
+# both the build log and build directory look nicer (toolchain-virtual instead
+# of toolchain-undefined)
 $(eval $(virtual-package))