Linux deb: Fix config fragment usage

[lool]

Fix a typo testing whether there is a local config fragment. This is
nasty in that it won't fail even under set -e as it's under an if
statement.

[lool]

I will look at adding shellcheck tests to prevent this in the future; shellcheck currently reports:

% shellcheck scripts/*

In scripts/build-linux-deb.sh line 77:
if [ -r kernel/configs/local.config]; then
^-- SC1009 (info): The mentioned syntax error was in this if expression.
   ^-- SC1073 (error): Couldn't parse this test expression. Fix to allow more checks.
        ^-- SC1019 (error): Expected this to be an argument to the unary condition.
                                    ^-- SC1020 (error): You need a space before the ].
                                    ^-- SC1072 (error): Missing space before ]. Fix any mentioned problems and try again.


In scripts/build-u-boot-rb1.sh line 75:
make -j`nproc` \
       ^-----^ SC2046 (warning): Quote this to prevent word splitting.
       ^-----^ SC2006 (style): Use $(...) notation instead of legacy backticks `...`.

Did you mean:
make -j$(nproc) \

For more information:
  https://www.shellcheck.net/wiki/SC1020 -- You need a space before the ].
  https://www.shellcheck.net/wiki/SC2046 -- Quote this to prevent word splitt...
  https://www.shellcheck.net/wiki/SC2006 -- Use $(...) notation instead of le...

and would have caught that; note that bash -n didn't see anything:

% bash -n scripts/*

[lool]

I'm going to self-approve this to reroll a kernel and unbreak the image builds – currently all images fail to boot because the kernel is built without CONFIG_EFI_ZBOOT.