toolchain: add .mchckrc support

The .mchckrc file can store common settings, such as SATDIR or
MCHCKADAPTER.  Use ?= assignment to allow temporary overrides, e.g.:

MCHCKADAPTER?=	name=mchck:dev=/dev/serial/by-id/usb-mchck.org_MC_HCK_SWD_adapter-if00
toolchain: add swd-flash make target
toolchain: start gdbserver on `make gdb'