[RCAR] disable a default config in kernel

[RCAR] disable a default config in kernel

The meta-rcar layer activates by default the configuration MTD_RENESAS_RPC_HYPERFLASH in kernel.
MTD_RENESAS_RPC_HYPERFLASH is not upstream, it's a meta-rcar specific feature.
MTD_RENESAS_RPC_HYPERFLASH feature in the kernel need an firmware update on the board and
can be considered (IMHO) as a security issues in production.

We need to set DISABLE_RPC_ACCESS to deactivate it.

v2 (jsmoeller): Use ?= for DISABLE_RPC_ACCESS .

Bug-AGL: SPEC-3674

Signed-off-by: ronan <ronan.lemartret@iot.bzh>
Change-Id: Iefaf9ca115d1fb805b0d3766fc0c800b74daecee
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl/+/25815
Tested-by: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
ci-image-build: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
ci-image-boot-test: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>