Parse else ifxxx statement

More recent versions of make provide "else ifdef/ifndef/ifeq/ifneq" directive:
ifdef FOO
...
else ifdef BAR
...
endif
Fix the parser to handle it. It returns the same Directive as the respective ifxxx
counterpart, only that the Name is set to "elifdef/elifndef/elifeq/elifneq".

Test: treehugger
Change-Id: I74c6a2c7224bce1dd3465012fa84880fae21349b