[linux] Remove Linux-specific code from default (non-Linux) build

Building the Linux-specific code (tap.o et al) requires external
headers that have proven to be extremely variable across systems,
causing frequent build failures.

Until this situation is rectified, remove the Linux-specific code from
the default (non-Linux build).

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/Makefile

 SRCDIRS		+= drivers/nvs
 SRCDIRS		+= drivers/bitbash
 SRCDIRS		+= drivers/infiniband
-SRCDIRS		+= drivers/linux
-SRCDIRS		+= interface/pxe interface/efi interface/smbios interface/linux
+SRCDIRS		+= interface/pxe interface/efi interface/smbios
 SRCDIRS		+= tests
 SRCDIRS		+= crypto crypto/axtls crypto/matrixssl
 SRCDIRS		+= hci hci/commands hci/tui

src/arch/x86/Makefile.linux

 MEDIA = linux
 
 INCDIRS += arch/x86/include/linux
+SRCDIRS += interface/linux
+SRCDIRS += drivers/linux
 SRCDIRS += arch/x86/core/linux
 
 $(BIN)/%.linux : $(BIN)/%.linux.tmp