[doc] Fix make commands in {fpga,sw} user guides
* Point to make -C sw/device, since this is where the device Make
scaffolding lives.
* Add explicit make boot_rom command in fpga getting started
instructions. Some users are running into problems due to the fact
that the boot_rom file dependency is not checked at FPGA bitstream
build time and the instructions don't explicitly call out the make
command for boot_rom.
Keeping instructions in Make format as requested by
lowrisc/opentitan#733.
diff --git a/doc/ug/getting_started_fpga.md b/doc/ug/getting_started_fpga.md
index d0c8100..fa0e921 100644
--- a/doc/ug/getting_started_fpga.md
+++ b/doc/ug/getting_started_fpga.md
@@ -34,6 +34,7 @@
```console
$ . /tools/xilinx/Vivado/2018.3/settings64.sh
$ cd $REPO_TOP
+$ make -C sw/device SW_DIR=boot_rom clean all
$ fusesoc --cores-root . build lowrisc:systems:top_earlgrey_nexysvideo
```
