- don't use undefined flag=0. It should be CONSTANT instead.
- don't allow 'UMat* m=NULL' argument (except LOCAL/CONSTANT flags).
  This case is not handled well to provide NULL __global pointers.
  It is better to use '-D' macro defines instead (at least for performance)

(2.4) Backports from master branch (#7854)

- added HAL documentation support
- added documentation to HAL replacement interface
- updated several HAL functions in imgproc module

- added new functions from core module: split, merge, add, sub, mul, div, ...
- added function replacement mechanism
- added example of HAL replacement library

fixed http://code.opencv.org/issues/3935
possibly fixed http://code.opencv.org/issues/3594

IPP can be switched on and off on runtime;

Optional implementation collector was added (switched off by default in CMake). Gathers data of implementation used in functions and report this info through performance TS;

TS modifications for implementations control;

1. someMatrix.data -> someMatrix.prt()
2. someMatrix.data + someMatrix.step * lineIndex -> someMatrix.ptr( lineIndex )
3. (SomeType*) someMatrix.data -> someMatrix.ptr<SomeType>()
4. someMatrix.data -> !someMatrix.empty() ( or !someMatrix.data -> someMatrix.empty() ) in logical expressions

NOT verified on target platform, compilation passes with and without NEON.

1. disabled OpenCL acceleration for cv::multiply() (CV_32F), cv::divide (CV_32F), cv::convertScaleAbs (CV_32F) and cv::reduce (SUM, CV_32F), cv::reduce (MIN & MAX), cv::flip (3-channel case).
2. changed the number of test loops from 1 to 30 (except for cv::pow() test, which fails for yet unknown reason)
3. disabled IPP acceleration for 3-channel norms.
4. modified relativeNorm test function to handle very small values