fix makefile inheritance system

The variable inheritance system can leave extra spaces in a variable
when multiple levels of inheritance are used.  It can expand:

  "@inherit:foo @inherit:bar @inherit:baz"

     to

  "value @inherit:bar @inherit:baz"

     and then

  "value  @inherit:baz"   (if bar has no value for the variable)

     and then

  "value "  (if baz also has no value).

Call strip to remove excess spaces between words after each
replacement pass.

Change-Id: Ic4cfc6fed1865066a6353a522e4bdcb68ec875d3