wanted: specify single hard fork height in config.toml

[brilliant-lx]

Rationale

There is a configure section in config.toml, like:

[Eth.Genesis.Config]
ChainID = 56
HomesteadBlock = 0
...
GibbsBlock = 23846001
PlanckBlock = 27281024

Currently, if you download the mainnet.zip or testnet.zip from https://github.com/bnb-chain/bsc/releases
, unzip it, then you get the config.toml without the above config section.
People can run full sync using this config.toml, since the default block height for each hard fork have been defined in code: https://github.com/bnb-chain/bsc/blob/master/params/config.go#L172

But if people setup a single hard fork height, then the full sync could break, like:

[Eth.Genesis.Config]
ChainID = 56
PlanckBlock = 27281024

That is people can either skip the section or set all the hard forks, but can not just set a part of the fork height, it is not reasonable.
The expected behaviors should: if the dedicated fork height is not configured in config.toml, it should use the default value in code: https://github.com/bnb-chain/bsc/blob/master/params/config.go#L172

Implementation

Just to fix it.