scripts/nuttx2ffff: fix usage
scripts/nuttx2ffff: fix usage

The current usage printing is a bit confusing, misses one argument,
and doesn't use some best practices.

Clean up a bit:

- Put usage in a function so it can be called from elsewhere

- Delete the comment which replicates the usage to reduce repetition and
  opportunities for the two to drift

- Get the name of the program from $0 rather than hard-coding it in the
  script

- Say that the nuttx file must be in ELF format, so people don't try
  to use a .bin

- Name the path the nuttx file comes from, so people know the
  environment variable to set

- Add missing usage information for the "--outdir" argument

Testing Done: Run "nuttx2fff --help", see desired usage message
Bug Number: SW-2744
Review URL: https://reviews.leaflabs.com/r/2747/
Reviewed by: Eli Sennesh, Mark Greer

Signed-off-by: Marti Bolivar <35e491f5b70a779e5455b1f9617e0073292369ca@leaflabs.com>

projectara/bootrom-tools,gregkh/bootrom-tools,projectara/bootrom-tools,gregkh/bootrom-tools,projectara/bootrom-tools,gregkh/bootrom-tools