Makefile: make clean should delete include/generated/env.in

'make sifive_unamtched_defconfig; make clean; make' fails if file
include/generated/env.in exists. 'make clean' should remove all files that
stop building.

Add file include/generated/env.in to the clean target.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Marek Behún <marek.behun@nic.cz>