bootconfig: Fix testcase to increase max node

bootconfig: Fix testcase to increase max node

Since commit 6c40624930c5 ("bootconfig: Increase max nodes of bootconfig
from 1024 to 8192 for DCC support") increased the max number of bootconfig
node to 8192, the bootconfig testcase of the max number of nodes fails.
To fix this issue, we can not simply increase the number in the test script
because the test bootconfig file becomes too big (>32KB). To fix that, we
can use a combination of three alphabets (26^3 = 17576). But with that,
we can not express the 8193 (just one exceed from the limitation) because
it also exceeds the max size of bootconfig. So, the first 26 nodes will just
use one alphabet.

With this fix, test-bootconfig.sh passes all tests.

Link: https://lore.kernel.org/all/167888844790.791176.670805252426835131.stgit@devnote2/
Reported-by: Heinz Wiesinger <pprkut@slackware.com>
Link: https://lore.kernel.org/all/2463802.XAFRqVoOGU@amaterasu.liwjatan.org
Fixes: 6c40624930c5 ("bootconfig: Increase max nodes of bootconfig from 1024 to 8192 for DCC support")
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>