smbios: empty strings in smbios_add_string()

smbios_add_string() cannot deal with empty strings. This leads to incorrect
property values and invalid tables. E.g. for the pine64-lts_defconfig
CONFIG_SMBIOS_MANUFACTURER="". Linux command dmidecode shows:

Table 1:
    Manufacturer: sunxi
    Product Name: sunxi

Table 3:
    Invalid entry length (2). DMI table is broken! Stop.

Replace empty strings by "Unknown".

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>