Restore custom entropy option

[jaekwon]

From e0c38b3#diff-994412d86e91569aed9057984c68a54e :

remove custom entropy option; nobody uses it

Somebody uses it. Just saying.

[ecdsa]

what is your use case?

[RealBTC]

I back this, if PC rng is weak it is logical to have the option to have more entropy. The 12 word seeds since 2.7 are too weak, but atleast have the option to have more entropy.

[ecdsa]

ok, I restored it. I guess it is cheaper to have it than to educate users about entropy.