dropbear: adjust failsafe script

dropbear: adjust failsafe script

- try to detect supported (hostkey) algorithms; otherwise fallback to predefined list;
- remove size constraint for ECDSA: custom build may include only 384 or 521 bit curves;
- remove size constraint for RSA: default RSA key size is 2048 bits which is sufficient for SSH security recommendations, and previous value of 1024 bits is considered insecure.

Signed-off-by: Konstantin Demin <rockdrilla@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/23217
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>