treewide: drop redundant "type string" for SYS_SOC and friends

treewide: drop redundant "type string" for SYS_SOC and friends

The Kconfig symbols SYS_ARCH, SYS_CPU, SYS_SOC, SYS_VENDOR and
SYS_BOARD are defined in arch/Kconfig as having type string, and most
board files simply amend those definition with suitable

  default "foo"

or

  default "foo" if BAR

stanzas. But some also include a redundant repetition of the type.

Homogenize the code base by removing those lines.

Generated by

  find arch/*/ board -name Kconfig | xargs perl -i -g -pe 's/(config SYS_(ARCH|CPU|SOC|VENDOR|BOARD)\n)\s*string\n/\1/gs'

with the trailing slash in arch/*/ ensuring that arch/Kconfig itself
is not found.

This does not change boards which add a prompt string, e.g.

  string "Board name"

because I think those change the semantics of the symbol into being
user-settable.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Reviewed-by: Tom Rini <trini@konsulko.com>