ARMv7-M: use command handler for "dap baseaddr".

Make the ARMv7-M DAP code reuse the command handler for "dap baseaddr".
For some reason, this DAP command wasn't converted earlier.

This is a code shrink and simplification; it also removes a needless
transport dependency on JTAG.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>