toolchain: invert glibc <-> !static dependency

Currently, glibc depends on !BR2_STATIC_LIBS in all the toolchain
variants.

However, for some architectures, glibc is the only supported libc. In
commit 3b3105328e ("Config.in: only
allow BR2_STATIC_LIBS on supported libc/arch"), we implemented a fix
to avoid configurations were BR2_STATIC_LIBS=y with an architecture
already supported by glibc, because these configurations are
impossible. This commit 3b3105328e
prevents from selecting BR2_STATIC_LIBS=y when the C library used for
the internal toolchain backend is glibc.

However, it introduces a discrepency between how this topic is handled
for internal and external toolchains:

 - For internal toolchains, we prevent BR2_STATIC_LIBS=y if glibc is
   chosen.

 - For external toolchains, we allow BR2_STATIC_LIBS=y in all cases,
   and it's each glibc toolchain that has !BR2_STATIC_LIBS

This commit addresses this discrepency by preventing BR2_STATIC_LIBS=y
if glibc is chosen in all cases.

Thanks to this, we can remove the !BR2_STATIC_LIBS dependency on both
the glibc package, and all glibc external toolchains.

Fixes: https://bugs.busybox.net/show_bug.cgi?id=14256

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
[Thomas: update to master, fix the gen-bootlin-toolchains script, add
a comment in the static/shared choice to indicate that static is
supported only with uclibc or musl]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

support/scripts/gen-bootlin-toolchains

@@ -326,8 +326,6 @@ class Toolchain:
                 # glibc needs mmu support
                 if "BR2_USE_MMU" not in depends:
                     depends.append("BR2_USE_MMU")
-                # glibc doesn't support static only configuration
-                depends.append("!BR2_STATIC_LIBS")
                 selects.append("BR2_TOOLCHAIN_EXTERNAL_GLIBC")
             elif frag.startswith("BR2_TOOLCHAIN_EXTERNAL_CUSTOM_MUSL"):
                 # musl needs mmu support
@@ -501,8 +499,6 @@ def gen_config_in_options(toolchains, fpath):
         f.write("\tbool\n")
         for arch, details in arches.items():
             conditions = details['conditions'].copy()
-            if set([t.libc for t in toolchains if t.arch == arch]) == set(['glibc']):
-                conditions.append("!BR2_STATIC_LIBS")
             f.write("\tdefault y if %s\n" % " && ".join(conditions))
         f.write("\n")