mk: pass KERNEL as environment variable

This is a follow-up fix for "mk/ld.mk: trigger kernel-dependent
ld.lib.so build", which alleviates the need to specify the KERNEL at the
build-system command line. Thanks to Piotr Tworek for reporting.

Issue #4320

tool/builddir/build.mk

@@ -66,6 +66,7 @@ export LIB_DEP_FILE     ?= var/libdeps
 export ECHO             ?= echo -e
 export CONTRIB_DIR
 export BOARD
+export KERNEL
 
 #
 # Convert user-defined directories to absolute directories