linux: Use SUMMARY to describe the linux provider

Replace DESCRIPTION to SUMMARY on linux recipes.

The SUMMARY content may be used in Release Notes to
describe the each kernel provider.

Add SUMMARY for linux-congatec, linux-cubox-i and
linux-timesys. For those, the SUMMARY used was the one
already in Release Notes [1] in file [2]

[1] https://github.com/Freescale/Documentation
[2] release-notes/source/kernel-linux.rst

Signed-off-by: Daiane Angolini <daiane.angolini@freescale.com>