build: use nproc instead of nproc - 2

Someone was trying to build on a 1 core machine and it broke the build.

The rationale for the -2 was to leave 2 cores for your to use your computer still,
but let us trust the OS scheduler a bit more.

build

@@ -8,7 +8,7 @@ configure=true
 config_fragments='br2'
 extra_make_args=
 gem5=false
-j="$(($(nproc) - 2))"
+j="$(nproc)"
 linux_reconfigure=false
 linux_kernel_custom_config_file=
 post_script_args=