readme: Update build instructions

README.md

@@ -20,9 +20,24 @@
 rustup target add riscv32imac-unknown-none-elf
 ```
 
+Either:
+
 - RISC-V toolchain ([e.g. from SiFive](https://static.dev.sifive.com/dev-tools/riscv64-unknown-elf-gcc-8.1.0-2019.01.0-x86_64-linux-ubuntu14.tar.gz))
 
-One of:
+or install prebuilt toolchain (easier):
+
+For Ubuntu/Debian:
+```
+sudo apt-get install gcc-riscv64-unknown-elf
+```
+
+For Fedora:
+
+```
+sudo dnf install binutils-riscv64-linux-gnu
+```
+
+For flashing, one of:
 
 - [dfu-util](http://dfu-util.sourceforge.net/)
 - [openocd for GD32VF103](https://github.com/riscv-mcu/riscv-openocd)
@@ -31,13 +46,6 @@ One of:
 
 When using `dfu-util`, the RISC-V toolchain is not necessary. Only `rust-objcopy` from [cargo-binutils](https://github.com/rust-embedded/cargo-binutils) is needed.
 
-Option B - Install prebuilt toolchain (easier):
-
-For Ubuntu/Debian:
-```
-sudo apt-get install gcc-riscv64-unknown-elf
-```
-
 ### Building 
 
 If you have a GD32VF103C**B** chip on your board, edit `.cargo/config.toml` and replace
@@ -48,6 +56,31 @@ To build all the provided examples run
 cargo build --examples --release --all-features
 ```
 
+#### Building everything including core
+
+An option for using only distribution packaging, no rustup, is to let the compiler pretend it's unstable and build the `core` crate.
+
+Install rust and its sources, and the lld linker.
+
+Fedora:
+
+```
+sudo dnf install rust rust-src lld
+```
+
+In `.cargo/config` add:
+
+```
+[target.riscv32imac-unknown-none-elf]
+linker = 'lld'
+```
+
+Use the following command to build:
+
+```
+RUSTC_BOOTSTRAP=1 cargo build -Z build-std --examples --release --all-features
+```
+
 ### Using dfu-util for Flashing
 
 The GD32VF103 contains a [DFU](https://www.usb.org/sites/default/files/DFU_1.1.pdf)