`command -v first second third` only works in `bash` and not in `sh`.
So replace with multiple calls to `command`.
This fixes using `objcopy` when the toolchain `objcopy` is not available.
This variable can be used to specify the linker which should be used to
link C++ object files. This is needed in order to use g++ for linking
with TOOLCHAIN=llvm.
- Merge flash and flash-elf commands since they were identical except
for the file name of the image
- Split GDB command from DBG environment variable to allow more easily
configure front-ends for GDB via environment variables.
- Remove verbose tests of empty variables and replace by `: ${VAR:=default}`
- Remove passed command line arguments to sub-functions, they were
unused in the functions anyway.
- Remove TUI variable, use `export DBG_EXTRA_FLAGS=-tui` to get the same
result.
