[scripts] Introduce scripts/fetch-toolchains.py

This change introduces $LK_ROOT/toolchain as an official installation
directory for toolchains, as well as a new script for conveniently
installing them there (versus manual GETs + untarring).

getting_started.md is updated to suggest its use.

docs/getting_started.md

@@ -6,25 +6,23 @@
 ```
 mkdir -p lk-work && cd lk-work
 ```
-2- Clone the repo
+2- Clone the repo and change dir to the root
 ```
 git clone https://github.com/littlekernel/lk
-```
-3- Download appropriate toolchain and extract it
-```
-wget https://newos.org/toolchains/riscv64-elf-14.2.0-Linux-x86_64.tar.xz
+cd lk
 
-mkdir -p toolchain
-tar xf riscv64-elf-14.2.0-Linux-x86_64.tar.xz
-cd ..
+```
+3- Download appropriate toolchain
+```
+# Fetches the latest riscv64-elf toolchain for your host.
+scripts/fetch-toolchains.py --prefix riscv64-elf
 ```
 4- Add toolchain to PATH
 ```
 export PATH=$PWD/toolchain/riscv64-elf-14.2.0-Linux-x86_64/bin:$PATH
 ```
-5- Change dir to lk to build and find available project
+5- Find available project
 ```
-cd lk
 ls project/*
 ```
 6- E.g pick `qemu-virt-riscv64-test` and build kernel