Add --git-describe and --git-rev CLI args

[sbellem]

Enable reproducible builds without git state by accepting version info as CLI arguments:

• --git-describe: version string for signing (e.g., v0.10.0-19-g0d934e1)
• --git-rev: commit hash for swap nonce (last 16 hex chars used)

Renames xtask --gitrev to --git-describe for clarity.

Motivation

Build systems like Guix and Nix operate in sandboxed environments without access to .git directories. Previously, we had to patch the code inline in Guix and Nix package definitions to inject version information at build time. These CLI arguments allow passing version info directly, eliminating the need for source patching.

Affected tools

Tool	New argument
xtask	--git-describe, --git-rev
xous-sign-image	--git-describe
xous-create-image	--git-rev
xous-app-uf2	--git-rev

Breaking change

The xtask --gitrev argument is renamed to --git-describe to better reflect its purpose (it accepts git describe output, not a commit hash).

Usage

# Get version info before entering sandboxed build                                                                                                                                                                                   
GIT_DESCRIBE=$(git describe --long --abbrev=9)                                                                                                                                                                                       
GIT_REV=$(git rev-parse HEAD)                                                                                                                                                                                                        
                                                                                                                                                                                                                                       
# Build without git state                                                                                                                                                                                                            
cargo xtask boot0 --git-describe "$GIT_DESCRIBE" --git-rev "$GIT_REV"                                                                                                                                                                

Testing

• cargo check passes
• Full build with explicit version args
• Verify signed images contain correct version
• Verify swap nonce matches expected value

[bunnie]

This will be handy for out of tree builds too - if i want to specify a git rev for those. The bin tool set is some of the gnarliest old code in the xous code base, sorry you had to slog through it.