Follow the target kernel's module hash algorithm

[xuzhen]

Fixes #266

[mavoga]

I would have preferred

sign_hash="$(grep "^CONFIG_MODULE_SIG_HASH=" "${kernel_config}" | cut -d= -f2 | tr -d '"')"

or even

eval " $(grep "^CONFIG_MODULE_SIG_HASH=" "${kernel_config}")"
sign_hash="${CONFIG_MODULE_SIG_HASH:-sha512}"
unset CONFIG_MODULE_SIG_HASH

My 2 cents

[evelikov]

@mavoga I'm open for the former, but I would avoid eval unless strictly needed. PRs welcome o/