linux: split calling "all" and "$(LINUX_TARGET_NAME)" targets

linux: split calling "all" and "$(LINUX_TARGET_NAME)" targets

In commit ffbe46a5295ce4e0442728f2ad3e19afa7eaa8ed ("linux: simplify
LINUX_BUILD_CMDS"), we changed LINUX_BUILD_CMDS to essentially do:

  make all $(LINUX_TARGET_NAME)

Unfortunately, it turns out that it breaks the build of a number of
defconfigs, with errors such as:

  fixdep: error opening file: arch/xtensa/boot/lib/.inftrees.o.d: No such file or directory

Calling "all" and "$(LINUX_TARGET_NAME)" as separate make invocations
avoids this problem, and fixes the build of several defconfigs.

Fixes:

  ts7680_defconfig
  https://gitlab.com/buildroot.org/buildroot/-/jobs/199339583

  qemu_xtensa_lx60_defconfig
  https://gitlab.com/buildroot.org/buildroot/-/jobs/199339553

  roseapplepi_defconfig
  https://gitlab.com/buildroot.org/buildroot/-/jobs/199339564

  qemu_xtensa_lx60_nommu_defconfig
  https://gitlab.com/buildroot.org/buildroot/-/jobs/199339554

  qemu_ppc64_e5500_defconfig
  https://gitlab.com/buildroot.org/buildroot/-/jobs/199339539

  freescale_t2080_qds_rdb_defconfig
  https://gitlab.com/buildroot.org/buildroot/-/jobs/199339455

  arcturus_ucp1020_defconfig
  https://gitlab.com/buildroot.org/buildroot/-/jobs/199339399

Cc: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Peter Korsgaard <peter@korsgaard.com>
Cc: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
[Thomas: add comment in the code to explain why we call the two make
targets separately, as suggested by Yann E. Morin]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>