utils/checkpackagelib/lib_mk.py: handle 'else' and 'elif' statements

utils/checkpackagelib/lib_mk.py: handle 'else' and 'elif' statements

An 'else' or 'elif' clause inside a make conditional should not be indented
in the same way as the if/endif clause. check-package did not recognize the
else statement and expected an indentation.

For example:

ifdef FOOBAR
	interesting
else
	more interesting
endif

would, according to check-package, need to become:

ifdef FOOBAR
	interesting
	else
	more interesting
endif

Treat 'else' and 'elif' the same as if-like keywords in the Indent test, but
take into account that 'else' is also valid shell, so we need to correctly
handle line continuation to prevent complaining about the 'else' in:

ifdef FOOBAR
	if true; \
	    ... \
	else \
	    ... \
	fi
endif

We don't add the 'else' and 'elif' statements to start_conditional, because
it would cause incorrect nesting counting in class OverriddenVariable.

Signed-off-by: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>