.gitignore: add SPDX License Identifier

Add SPDX License Identifier to all .gitignore files.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Update .gitignore files for generated targets

The generated 'capflags.c' file wasn't properly ignored, and the list of
files in scripts/basic/ wasn't up-to-date.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>