u-boot-update: use /etc/kernel/cmdline instead of /etc/default/u-boot
With the previous implementation, we used both the contents of
/etc/kernel/cmdline and of
/etc/default/u-boot, which could cause
some command-line parameters to appear twice on the final command-line.
As the preferred method is to use
/etc/kernel/cmdline, this commit
improves on the previous one by using only the contents of this file,
and revert to using the variables in
/etc/default/u-boot in case
cmdline is missing or empty.
Signed-off-by: Arnaud Ferraris email@example.com