dropbear: more options, option dependencies, detailed help, 2 patches

Changes:
- added two new patches from upstream
- new menuconfig structure
- added in menuconfig almost all available configuration options
- added dependencies for options (some are unavailable on musl)
- added warning messages for invalid configs
- all options have detailed help descriptions
- removed or replaced all options that required editing sysoptions.h

Signed-off-by: Marius Dinu <[email protected]>