mesh: Move commonly used constants into mesh-defs.h

mesh: Move commonly used constants into mesh-defs.h

This consolidates definitions of commonly used constant values
into a single header file. The constant values are based on mesh
specification, plus a few internal utility constants (masks, etc.)

Also, removes redundant redefinition maximum message length.

Change-Id: Ie6c8d26b72f7ff80078a9f96119f37508d1fdc72
Signed-off-by: Anupam Roy <anupam.r@samsung.com>