linux: Add version 4.14.48

The change-logs are available online. Several security fixes went in.

```
$ grep . /sys/devices/system/cpu/vulnerabilities/
/sys/devices/system/cpu/vulnerabilities/meltdown:Mitigation: PTI
/sys/devices/system/cpu/vulnerabilities/spec_store_bypass:Vulnerable
/sys/devices/system/cpu/vulnerabilities/spectre_v1:Mitigation: __user pointer sanitization
/sys/devices/system/cpu/vulnerabilities/spectre_v2:Mitigation: Full generic retpoline
```

Patch the build system, as the build fails with the currently installed
GNU Make 4.2.1+ [1].

[1]: https://patchwork.kernel.org/patch/10328633/