support/config-fragments/autobuild: use Bootlin toolchain for Xtensa uclibc

support/config-fragments/autobuild: use Bootlin toolchain for Xtensa uclibc

Instead of using an external toolchain built specifically for the
autobuilders to test Xtensa/uclibc, use a pre-built Bootlin toolchain.

To be noted: that fragment was in fact already using a Bootlin
bleeding-edge toolchain, because BR2_TOOLCHAIN_EXTERNAL_CUSTOM=y is
missing from the fragment:

    $ cat support/config-fragments/autobuild/br-xtensa-full.config >.config

    $ make olddefconfig

    $ grep BOOTLIN .config
    BR2_TOOLCHAIN_EXTERNAL_BOOTLIN=y
    BR2_TOOLCHAIN_EXTERNAL_BOOTLIN_ARCH_SUPPORTS=y
    BR2_TOOLCHAIN_EXTERNAL_BOOTLIN_XTENSA_LX60_UCLIBC_BLEEDING_EDGE=y
    # BR2_TOOLCHAIN_EXTERNAL_BOOTLIN_XTENSA_LX60_UCLIBC_STABLE is not set

The original fragment was supposed to use a stable toolchain, so we
switch to explictly use a stable Bootlin toolchain.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
[yann.morin.1998@free.fr:
  - add blurb about missing BR2_TOOLCHAIN_EXTERNAL_CUSTOM=y
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>