The current assert_boolean does not work with variables assigned with
'=' flavor instead of ':='.
For example,
FOO = $(BAR)
BAR := 1
Here, $(value FOO) is evaluated to $(BAR), not 1. This is not what
we expect. While I am here, I simplified the implementation.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
be4cd40e8b