Derive varlog size based on total flash size

[prsunny]

- What I did
Logic added to derive var/log size based on flash size and not check for platform

- How I did it
Check for flash_size and assign varlog_size

- How to verify it
Run the following command

admin@7060cx:~$ df -h
Filesystem      Size  Used Avail Use% Mounted on
/dev/loop1      380M  2.9M  353M   1% /var/log

Prior to this change,

admin@7060cx:~$ df -h
Filesystem      Size  Used Avail Use% Mounted on
/dev/loop1       93M   56M   30M  66% /var/log

- Description for the changelog
Derive varlog_size based on flash_size

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