No need to override mkshrc as it no longer exports PS1

busybox sh works fine with the one in android_external_mksh

Change-Id: I09ae2d16837053ea0be50d3320eccf47057e25c1
vendor/mk: merge CyanogenMod

Change-Id: I5af6473547d1e8efb2f4cc3f6989b67cc0e889aa