Add support for slackware distribution (linux_install.sh) (qmk#4369)

* Add support for slackware distribution (linux_install.sh)

The required packages are not provided by the official repository. The packages
need to be installed from slackbuilds.org either manually or with the help of
third party tools like sbotools.

* Modify linux_install.sh according to code review

Modifies the Slackware section of the linux_install.sh script based on the
suggestions from the code review.

* Modify utils/linux_install.sh according to suggestions

util/linux_install.sh

@@ -4,6 +4,8 @@
 
 GENTOO_WARNING="This script will make a USE change in order to ensure that that QMK works on your system. All changes will be sent to the the file /etc/portage/package.use/qmk_firmware -- please review it, and read Portage's output carefully before installing any packages on your system. You will also need to ensure that your kernel is compiled with support for the keyboard chip that you are using (e.g. enable Arduino for the Pro Micro). Further information can be found on the Gentoo wiki."
 
+SLACKWARE_WARNING="You will need the following packages from slackbuilds.org:\n\tarm-binutils\n\tarm-gcc\n\tavr-binutils\n\tavr-gcc\n\tavr-libc\n\tavrdude\n\tdfu-programmer\n\tdfu-util\n\tnewlib\nThese packages will be installed with sudo and sboinstall, so ensure that your user is added to sudoers and that sboinstall is configured."
+
 if grep ID /etc/os-release | grep -qE "fedora"; then
 	sudo dnf install \
 		arm-none-eabi-binutils-cs \
@@ -126,6 +128,26 @@ elif grep ID /etc/os-release | grep -qE "opensuse|tumbleweed"; then
 		wget \
 		zip
 
+elif grep ID /etc/os-release | grep -q slackware; then
+	printf "$SLACKWARE_WARNING\n"
+	printf "\nProceed (y/N)? "
+	read -r answer
+	if echo "$answer" | grep -iq "^y" ;then
+		sudo sboinstall \
+			avr-binutils \
+			avr-gcc \
+			avr-libc \
+			avrdude \
+			dfu-programmer \
+			dfu-util \
+			arm-binutils \
+			arm-gcc \
+			newlib
+		echo "Done!"
+	else
+		echo "Quitting..."
+	fi
+
 else
 	echo "Sorry, we don't recognize your OS. Help us by contributing support!"
 	echo