[check-config] No warning in blank argument
[check-config] No warning in blank argument

If user run current script whthout argument, the script will search
config in default dir list, but output following message:
 | # script/check-config.sh
 | warning: /proc/config.gz seems not a kernel config, searching other paths for kernel config ...
            ^^^^^^^^^^^^^^^
 | info: reading kernel config from /boot/config-4.7.0_HEAD_523d939ef98fd712632d93a5a2b588e477a7565e_ ...
 | ...

We can make output better by restruct the code struct:
1: Specify nothing
   Show info, and search default dir
2: Specify a config file
   Use it directly
3: Specify a wrong config file
   Show warning, and search default dir
4: Specify a dir
   Info, and search specified dir

Test:
 | # script/check-config.sh
 | info: no config specified, searching for kernel config ...
 | info: reading kernel config from /boot/config-4.7.0_HEAD_523d939ef98fd712632d93a5a2b588e477a7565e_ ...
 |
 | # script/check-config.sh /linux/.config
 | info: reading kernel config from /linux/.config ...
 |
 | # script/check-config.sh /linux/.configgg
 | warning: /linux/.configgg seems not a kernel config, searching other paths for kernel config ...
 | info: reading kernel config from /boot/config-4.7.0_HEAD_523d939ef98fd712632d93a5a2b588e477a7565e_ ...
 |
 | # script/check-config.sh /linux
 | info: input is a directory, searching for kernel config in this directory...
 | info: reading kernel config from /linux/.config ...
 |

Signed-off-by: Zhao Lei <aacdd228f50c574b28b524b23f12fb9d52d6e0e3@cn.fujitsu.com>

maxamillion/runc,adrianreber/runc,rhatdan/runc,estesp/runc,cyphar/runc,justincormack/runc,stefanberger/runc,mYmNeo/runc,vbatts/runc,wking/runc,WeiZhang555/runc,adrianreber/runc,sunyuan3/runc,maxamillion/runc,WeiZhang555/runc,shishir-a412ed/runc,opencontainers/runc,justincormack/runc,avagin/runc,kolyshkin/runc,xiaochenshen/runc,vbatts/runc,cloudfoundry-incubator/runc,opencontainers/runc,cloudfoundry-incubator/runc,coolljt0725/runc,hqhq/runc,jhowardmsft/runc,jhowardmsft/runc,justincormack/runc,stefanberger/runc,dqminh/runc,mikebrow/runc,rajasec/runc,crosbymichael/runc,dqminh/runc,cyphar/runc,dqminh/runc,crosbymichael/runc,estesp/runc,docker/runc,mheon/runc,shishir-a412ed/runc,laijs/runc,opencontainers/runc,cloudfoundry-incubator/runc,mYmNeo/runc,stefanberger/runc,kolyshkin/runc,mheon/runc,mikebrow/runc,crosbymichael/runc,hqhq/runc,sunyuan3/runc,sunyuan3/runc,xiaochenshen/runc,vbatts/runc,rhatdan/runc,rajasec/runc,docker/runc,wking/runc,coolljt0725/runc,mheon/runc,mYmNeo/runc,avagin/runc,maxamillion/runc,shishir-a412ed/runc,laijs/runc,wking/runc,estesp/runc,rhatdan/runc,mikebrow/runc,kolyshkin/runc,laijs/runc,avagin/runc,cyphar/runc,opencontainers/runc,hqhq/runc,WeiZhang555/runc,rajasec/runc,coolljt0725/runc,adrianreber/runc,dqminh/runc,rhatdan/runc,kolyshkin/runc,docker/runc,jhowardmsft/runc,xiaochenshen/runc