Unit testing framework and devicetree framework refactoring (#50)

* re-write the fdt iterator and parsing procedure

* unit test framework and devtree testings

* add expect_* assertions, test statistics, more dt test cases

* basic.dts : basic dt property testing: trivial, simple prop_encoded,
  mixed prop_encoded
* inheritence.dts : #address-cell, #size-cell and #interrupt-cell parent
  inheritence test.

* regression fix on dt, add dt prop extractor api (dtpx)

* tests for interrupt node and rework dtpi api

* add tests for interrupt node, exercise interrupt mask and interrupt map
* redesign the dtpi api to make it simpler to use
* rename dt_node_* and dt_prop_* struct to dtn_* and dtp_* to make it
  adhere to related functions

* struct dtspec: unified specifier-map representation

* add correpsond unit test.

* fix minor issues, integrate the test into main makefile

* api: dt_interrupt_at to query interrupt specifier at given location

* add unit test cases for interrupt node related test

* rewrite FDT iterator, enable in-place DT lookup

* add alloc-less iterator for /memory, /reserved-memory nodes
* unit tests for the rewrited iterators

* unit test for fdt-memscan

* fix detected issues

* fix indentations