package/linux-headers: apply all Linux patches when BR2_KERNEL_HEADERS_AS_KERNEL=y

When BR2_KERNEL_HEADERS_AS_KERNEL=y, we expect that the Linux kernel
headers code will be exactly the same as the Linux kernel code
itself. The code currently takes into account the patches defined by
BR2_LINUX_KERNEL_PATCH, but not the kernel patches that are stored in
linux's BR2_GLOBAL_PATCH_DIR.

So for example, the current qemu_riscv32_virt_defconfig has:

BR2_GLOBAL_PATCH_DIR="board/qemu/riscv32-virt/patches/"

With:

board/qemu/riscv32-virt/patches/
└── linux
    └── 0001-Revert-riscv-Use-latest-system-call-ABI.patch

This patch gets properly applied when the Linux kernel is built, but
not when the linux-headers package is built.

This commit fixes that by making sure patches stored in the "linux"
BR2_GLOBAL_PATCH_DIR subdirectory are taken into account.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Acked-by: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>