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>