minikconfig: add AST

minikconfig: add AST

Add Python classes that represent the Kconfig abstract syntax tree.
The abstract syntax tree is stored as a list of clauses.  For example:

    config FOO
        depends on BAR
        select BAZ

is represented as three clauses:

    FOO depends on BAR
    FOO default n
    select BAZ if FOO

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20190123065618.3520-24-yang.zhong@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>