kconfig: streamline_config.pl: check defined(ENV variable) before using it

A user reported:
'Use of uninitialized value $ENV{"LMC_KEEP"} in split at
 ./scripts/kconfig/streamline_config.pl line 596.'

so first check that $ENV{LMC_KEEP} is defined before trying
to use it.

Fixes: c027b02d ("streamline_config.pl: add LMC_KEEP to preserve some kconfigs")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>