package/rocksdb: switch to generic-package

package/rocksdb: switch to generic-package

Switch from cmake-package to generic-package to allow rocksdb to run the
./build_tools/build_detect_platform script and detect compiler options
such as C++17 support for -faligned-new

First patch needs to be updated and second patch can be dropped

Fixes:
 - http://autobuild.buildroot.org/results/22c9909c0d20e3871775f3874f7723910d7e5a41
 - http://autobuild.buildroot.org/results/ab7b2bc9e9653a7093d8b27d4445c28993572ca4

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>