buildsystem: export TOOL_PREFIX

Signed-off-by: Andrew 'Necromant' Andrianov <andrew@ncrmnt.org>
deploy: support gdb debugging for stlink

Signed-off-by: Andrew 'Necromant' Andrianov <andrew@ncrmnt.org>
gcc: add kconfig option for -g flag

Signed-off-by: Andrew 'Necromant' Andrianov <andrew@ncrmnt.org>