kernel: don't fail if Modules.symvers doesn't exist

Kernels that do not use modules do not have the Modules.symvers file,
which causes the previous one-liner to fail.  Invert the logic so that
the absence of the Modules.symvers is a passing situation but we still
get failure checking on the install operation.

Signed-off-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>