toolchain.inc: Set OBJDUMP to empty string for sdk-cross==mingw
toolchain.inc: Set OBJDUMP to empty string for sdk-cross==mingw

A problem occurs when building a mingw32 sdk. This problem still needs to
be addresses. This fix, makes it possible to build that sdk.

hundeboll/core,sknsean/core,sknsean/core,sknsean/core,hundeboll/core,diorcety/oe-lite-core,sknsean/core,hundeboll/core,hundeboll/core,diorcety/oe-lite-core,sknsean/core