configure: do not use '@' in version string

Using an '@' in the version string breaks the gcc/glibc/gdb manuals.
Use a scheme similar to Debian, with a plus '+' instead of the '@',
as suggested by Michael Hope.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

# Blackfin specific configuration file

## select ARCH_SUPPORTS_32

## select ARCH_DEFAULT_32

## select ARCH_DEFAULT_LE

## select ARCH_SUPPORT_ARCH

## select ARCH_SUPPORT_CPU

## select ARCH_SUPPORT_TUNE

## select ARCH_SUPPORT_FPU

##

## help The Blackfin architecture