am37x-evm.conf: Use '+=' not '?=' in IMAGE_FSTYPES
am37x-evm.conf: Use '+=' not '?=' in IMAGE_FSTYPES

As per
http://lists.linuxtogo.org/pipermail/openembedded-core/2012-March/020053.html
a machine conf file should use '+=' to set IMAGE_FSTYPES.

Signed-off-by: Tom Rini <47764aff062bf9ebe98a2274b29052b74a9567e2@ti.com>
Signed-off-by: Denys Dmytriyenko <d29de71aea38aad3a87d486929cb0aad173ae612@ti.com>

tylerwhall/meta-ti,joelagnel/meta-ti,rvfh/meta-ti,treideme/meta-ti-rt,jumpnow/meta-ti,smoosvi/Test4,treideme/meta-ti-rt,joelagnel/meta-ti,rvfh/meta-ti,floft/meta-ti,rcn-ee/meta-ti,tylerwhall/meta-ti,smoosvi/Test4,rvfh/meta-ti,joelagnel/meta-ti,rcn-ee/meta-ti,tylerwhall/meta-ti,tylerwhall/meta-ti,RedFIR/meta-ti,Angstrom-distribution/meta-ti,RedFIR/meta-ti,mrchapp/meta-ti-amsdk,Angstrom-distribution/meta-ti,jumpnow/meta-ti,rcn-ee/meta-ti,mrchapp/meta-ti-amsdk,rcn-ee/meta-ti,RedFIR/meta-ti,floft/meta-ti,RedFIR/meta-ti,joelagnel/meta-ti