common: Drop LOGLEVEL to 4

While this came in with a default value of 6 I am lowering this to 4.
The MTD/UBI code has a large number of error messages that we include
now.  In addition, "normally" warning messages are not included so this
feels like a more natural level to have.

Signed-off-by: Tom Rini <trini@konsulko.com>