[OE-core] Unable to build rootfs using RPM packages when TARGET_VENDOR is unset

[John Ernberg]

Hi

I found a problem that prevents me from building a rootfs using RPM 
packages when the variable TARGET_VENDOR is unset (overridden to 
TARGET_VENDOR = "").

I traced it as far as Smart / RPM rejecting all the built arches with 
score 0 and thus creating an empty package cache, that the cause is the 
lines in the platform file that's generated for Smart.

The platform file is generated to contain a lot of lines on the format 
"armv7ahf_neon-.*-linux.*", when TARGET_VENDOR is unset however, the 
platform will be named "armv7ahf_neon-linux" and the regex ends up not 
matching, the smallest possible string is "armv7ahf_neon--linux" which 
requires TARGET_VENDOR to be set to at least "-".

Unfortunately I never managed to figure out how to solve it and now I no 
longer have time to work on the issue, but I hope that my debugging of 
it may be useful, so I wanted to document it.

Best regards // John Ernberg