Fixes: 4fafb35d925e5 ("kbuild: cherry-pick changes from Linux v5.0-rc3")

The change cherry-picked from the Kernel commit 7d0ea2524202 ("kbuild: use
'else ifeq' for checksrc to improve readability") also removed the
trailing semicolons from the cmd_*checksrc lines. These were removed in
kernel commit e5d289100d ("kbuild: remove trailing semicolon from cmd_*
passed to if_changed_rule"), but seems the barebox Kbuild version is not
yet ready for this change. Without the semicolons a make C=1 build fails
with:

No such file: echo

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>