[SRC_C][U-BOOT] Fix repo firm in install.sh

Previously, the firm_sysdata_version function was called for each commit when patch files were created. As a result, it read data from the current working directory, which was the U-Boot repository rather than the intended SYSDATA repository.

This patch ensures the function is called only once at the beginning, before any operations are performed. The result is stored in a variable and reused in all commit messages, ensuring the data is consistently taken from the SYSDATA repository where the script is originally executed.

Signed-off-by: Davide Cardillo davide.cardillo@seco.com