Makefile: Conditionally add defaultenv_h to envtools target

When building the envtools target with CONFIG_USE_DEFAULT_ENV_FILE=y,
the tools require generated/defaultenv_autogenerated.h.

 In file included from tools/env/fw_env.c:126:
 include/env_default.h:115:10: fatal error: generated/defaultenv_autogenerated.h: No such file or directory
   115 | #include "generated/defaultenv_autogenerated.h"
       |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Joel Stanley <joel@jms.id.au>