external-toolchain: allow override of linux-libc-headers provider
external-toolchain: allow override of linux-libc-headers provider

only set PREFERRED_PROVIDER_linux-libc-headers if it is not
already assigned a value.

Signed-off-by: Ben Gardiner <70df8feb3ef5e98f393abcbbb2227739b5f9dc64@nanometrics.ca>
Tested-by: Andrea Galbusera <gizero@gmail.com>
Signed-off-by: Tom Rini <4c3d5c8f8c21c27dd329bd585f481113514551fd@mentor.com>
(cherry picked from commit 829ea5110bc82f71a225f168bcb6a9a327b1b5c8)

Signed-off-by: Ben Gardiner <70df8feb3ef5e98f393abcbbb2227739b5f9dc64@nanometrics.ca>
Acked-by: Denys Dmytriyenko <d29de71aea38aad3a87d486929cb0aad173ae612@ti.com>
Signed-off-by: Tom Rini <4c3d5c8f8c21c27dd329bd585f481113514551fd@mentor.com>

nx111/openembeded_openpli2.1_nx111,sledz/oe,mrchapp/arago-oe-dev,buglabs/oe-buglabs,SIFTeam/openembedded,SIFTeam/openembedded,SIFTeam/openembedded,nx111/openembeded_openpli2.1_nx111,BlackPole/bp-openembedded,dave-billin/overo-ui-moos-auv,buglabs/oe-buglabs,sledz/oe,sutajiokousagi/openembedded,sampov2/audio-openembedded,buglabs/oe-buglabs,sampov2/audio-openembedded,sutajiokousagi/openembedded,buglabs/oe-buglabs,nx111/openembeded_openpli2.1_nx111,mrchapp/arago-oe-dev,dave-billin/overo-ui-moos-auv,sledz/oe,BlackPole/bp-openembedded,dave-billin/overo-ui-moos-auv,sledz/oe,mrchapp/arago-oe-dev,SIFTeam/openembedded,dave-billin/overo-ui-moos-auv,sutajiokousagi/openembedded,BlackPole/bp-openembedded,mrchapp/arago-oe-dev,SIFTeam/openembedded,SIFTeam/openembedded,sampov2/audio-openembedded,BlackPole/bp-openembedded,BlackPole/bp-openembedded,sutajiokousagi/openembedded,sutajiokousagi/openembedded,nx111/openembeded_openpli2.1_nx111,sampov2/audio-openembedded,sampov2/audio-openembedded,buglabs/oe-buglabs,nx111/openembeded_openpli2.1_nx111,mrchapp/arago-oe-dev,buglabs/oe-buglabs,SIFTeam/openembedded,sledz/oe,sampov2/audio-openembedded,dave-billin/overo-ui-moos-auv,sutajiokousagi/openembedded,mrchapp/arago-oe-dev,nx111/openembeded_openpli2.1_nx111,dave-billin/overo-ui-moos-auv,nx111/openembeded_openpli2.1_nx111,sledz/oe,BlackPole/bp-openembedded,sutajiokousagi/openembedded,sledz/oe,nx111/openembeded_openpli2.1_nx111,BlackPole/bp-openembedded,mrchapp/arago-oe-dev,buglabs/oe-buglabs,buglabs/oe-buglabs,sampov2/audio-openembedded,dave-billin/overo-ui-moos-auv