checkpatch: Add check for space indentation in Kconfig

checkpatch: Add check for space indentation in Kconfig

U-Boot requires Kconfig options to be indented with tabs, whereas Linux
allows spaces. Add a U-Boot specific check to warn when spaces are used
for indentation in Kconfig files.

To ensure this check is executed, move the u_boot_line() invocation in
process() to occur before the valid source file check. Previously,
Kconfig files were skipped by the file extension filter before the
U-Boot specific checks could run.

Example warning:

WARNING: Kconfig indentation should use tabs
+    bool

Link: https://lore.kernel.org/u-boot/20251222162026.GA847766@bill-the-cat/
Signed-off-by: Kuan-Wei Chiu <visitorckw@gmail.com>
Reviewed-by: Tom Rini <trini@konsulko.com>