Build: fix assert_boolean implementation

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>